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Network Working Group                                           E. NebelRequest For Comments: 1867                                   L. MasinterCategory: Experimental                                 Xerox Corporation                                                           November 1995Form-based File Upload in HTMLStatus of this Memo   This memo defines an Experimental Protocol for the Internet   community.  This memo does not specify an Internet standard of any   kind.  Discussion and suggestions for improvement are requested.   Distribution of this memo is unlimited.1. Abstract   Currently, HTML forms allow the producer of the form to request   information from the user reading the form.  These forms have proven   useful in a wide variety of applications in which input from the user   is necessary.  However, this capability is limited because HTML forms   don't provide a way to ask the user to submit files of data.  Service   providers who need to get files from the user have had to implement   custom user applications.  (Examples of these custom browsers have   appeared on the www-talk mailing list.)  Since file-upload is a   feature that will benefit many applications, this proposes an   extension to HTML to allow information providers to express file   upload requests uniformly, and a MIME compatible representation for   file upload responses.  This also includes a description of a   backward compatibility strategy that allows new servers to interact   with the current HTML user agents.   The proposal is independent of which version of HTML it becomes a   part.2.  HTML forms with file submission   The current HTML specification defines eight possible values for the   attribute TYPE of an INPUT element: CHECKBOX, HIDDEN, IMAGE,   PASSWORD, RADIO, RESET, SUBMIT, TEXT.   In addition, it defines the default ENCTYPE attribute of the FORM   element using the POST METHOD to have the default value   "application/x-www-form-urlencoded".Nebel & Masinter              Experimental                      [Page 1]

RFC 1867             Form-based File Upload in HTML        November 1995   This proposal makes two changes to HTML:   1) Add a FILE option for the TYPE attribute of INPUT.   2) Allow an ACCEPT attribute for INPUT tag, which is a list of      media types or type patterns allowed for the input.   In addition, it defines a new MIME media type, multipart/form-data,   and specifies the behavior of HTML user agents when interpreting a   form with ENCTYPE="multipart/form-data" and/or <INPUT type="file">   tags.   These changes might be considered independently, but are all   necessary for reasonable file upload.   The author of an HTML form who wants to request one or more files   from a user would write (for example):    <FORM ENCTYPE="multipart/form-data" ACTION="_URL_" METHOD=POST>    File to process: <INPUT NAME="userfile1" TYPE="file">    <INPUT TYPE="submit" VALUE="Send File">    </FORM>   The change to the HTML DTD is to add one item to the entity   "InputType". In addition, it is proposed that the INPUT tag have an   ACCEPT attribute, which is a list of comma-separated media types.  ... (other elements) ...  <!ENTITY % InputType "(TEXT | PASSWORD | CHECKBOX |                         RADIO | SUBMIT | RESET |                         IMAGE | HIDDEN | FILE )">  <!ELEMENT INPUT - 0 EMPTY>  <!ATTLIST INPUT          TYPE %InputType TEXT          NAME CDATA #IMPLIED  -- required for all but submit and reset          VALUE CDATA #IMPLIED          SRC %URI #IMPLIED  -- for image inputs --          CHECKED (CHECKED) #IMPLIED          SIZE CDATA #IMPLIED  --like NUMBERS,                                  but delimited with comma, not space          MAXLENGTH NUMBER #IMPLIED          ALIGN (top|middle|bottom) #IMPLIED          ACCEPT CDATA #IMPLIED --list of content types          >Nebel & Masinter              Experimental                      [Page 2]

RFC 1867             Form-based File Upload in HTML        November 1995  ... (other elements) ...3.  Suggested implementation   While user agents that interpret HTML have wide leeway to choose the   most appropriate mechanism for their context, this section suggests   how one class of user agent, WWW browsers, might implement file   upload.3.1 Display of FILE widget   When a INPUT tag of type FILE is encountered, the browser might show   a display of (previously selected) file names, and a "Browse" button   or selection method. Selecting the "Browse" button would cause the   browser to enter into a file selection mode appropriate for the   platform. Window-based browsers might pop up a file selection window,   for example. In such a file selection dialog, the user would have the   option of replacing a current selection, adding a new file selection,   etc. Browser implementors might choose let the list of file names be   manually edited.   If an ACCEPT attribute is present, the browser might constrain the   file patterns prompted for to match those with the corresponding   appropriate file extensions for the platform.3.2 Action on submit   When the user completes the form, and selects the SUBMIT element, the   browser should send the form data and the content of the selected   files.  The encoding type application/x-www-form-urlencoded is   inefficient for sending large quantities of binary data or text   containing non-ASCII characters.  Thus, a new media type,   multipart/form-data, is proposed as a way of efficiently sending the   values associated with a filled-out form from client to server.3.3 use of multipart/form-data   The definition of multipart/form-data is included insection 7.  A   boundary is selected that does not occur in any of the data. (This   selection is sometimes done probabilisticly.) Each field of the form   is sent, in the order in which it occurs in the form, as a part of   the multipart stream.  Each part identifies the INPUT name within the   original HTML form. Each part should be labelled with an appropriate   content-type if the media type is known (e.g., inferred from the file   extension or operating system typing information) or as   application/octet-stream.Nebel & Masinter              Experimental                      [Page 3]

RFC 1867             Form-based File Upload in HTML        November 1995   If multiple files are selected, they should be transferred together   using the multipart/mixed format.   While the HTTP protocol can transport arbitrary BINARY data, the   default for mail transport (e.g., if the ACTION is a "mailto:" URL)   is the 7BIT encoding.  The value supplied for a part may need to be   encoded and the "content-transfer-encoding" header supplied if the   value does not conform to the default encoding.  [Seesection 5 of   RFC 1521 for more details.]   The original local file name may be supplied as well, either as a   'filename' parameter either of the 'content-disposition: form-data'   header or in the case of multiple files in a 'content-disposition:   file' header of the subpart. The client application should make best   effort to supply the file name; if the file name of the client's   operating system is not in US-ASCII, the file name might be   approximated or encoded using the method ofRFC 1522.  This is a   convenience for those cases where, for example, the uploaded files   might contain references to each other, e.g., a TeX file and its .sty   auxiliary style description.   On the server end, the ACTION might point to a HTTP URL that   implements the forms action via CGI. In such a case, the CGI program   would note that the content-type is multipart/form-data, parse the   various fields (checking for validity, writing the file data to local   files for subsequent processing, etc.).3.4 Interpretation of other attributes   The VALUE attribute might be used with <INPUT TYPE=file> tags for a   default file name. This use is probably platform dependent.  It might   be useful, however, in sequences of more than one transaction, e.g.,   to avoid having the user prompted for the same file name over and   over again.   The SIZE attribute might be specified using SIZE=width,height, where   width is some default for file name width, while height is the   expected size showing the list of selected files.  For example, this   would be useful for forms designers who expect to get several files   and who would like to show a multiline file input field in the   browser (with a "browse" button beside it, hopefully).  It would be   useful to show a one line text field when no height is specified   (when the forms designer expects one file, only) and to show a   multiline text area with scrollbars when the height is greater than 1   (when the forms designer expects multiple files).Nebel & Masinter              Experimental                      [Page 4]

RFC 1867             Form-based File Upload in HTML        November 19954.  Backward compatibility issues   While not necessary for successful adoption of an enhancement to the   current WWW form mechanism, it is useful to also plan for a migration   strategy: users with older browsers can still participate in file   upload dialogs, using a helper application. Most current web browers,   when given <INPUT TYPE=FILE>, will treat it as <INPUT TYPE=TEXT> and   give the user a text box. The user can type in a file name into this   text box. In addition, current browsers seem to ignore the ENCTYPE   parameter in the <FORM> element, and always transmit the data as   application/x-www-form-urlencoded.   Thus, the server CGI might be written in a way that would note that   the form data returned had content-type application/x-www-form-   urlencoded instead of multipart/form-data, and know that the user was   using a browser that didn't implement file upload.   In this case, rather than replying with a "text/html" response, the   CGI on the server could instead send back a data stream that a helper   application might process instead; this would be a data stream of   type "application/x-please-send-files", which contains:   * The (fully qualified) URL to which the actual form data should     be posted (terminated with CRLF)   * The list of field names that were supposed to be file contents     (space separated, terminated with CRLF)   * The entire original application/x-www-form-urlencoded form data     as originally sent from client to server.   In this case, the browser needs to be configured to process   application/x-please-send-files to launch a helper application.   The helper would read the form data, note which fields contained   'local file names' that needed to be replaced with their data   content, might itself prompt the user for changing or adding to the   list of files available, and then repackage the data & file contents   in multipart/form-data for retransmission back to the server.   The helper would generate the kind of data that a 'new' browser   should actually have sent in the first place, with the intention that   the URL to which it is sent corresponds to the original ACTION URL.   The point of this is that the server can use the *same* CGI to   implement the mechanism for dealing with both old and new browsers.   The helper need not display the form data, but *should* ensure that   the user actually be prompted about the suitability of sending the   files requested (this is to avoid a security problem with malicious   servers that ask for files that weren't actually promised by theNebel & Masinter              Experimental                      [Page 5]

RFC 1867             Form-based File Upload in HTML        November 1995   user.) It would be useful if the status of the transfer of the files   involved could be displayed.5.  Other considerations5.1 Compression, encryption   This scheme doesn't address the possible compression of files.  After   some consideration, it seemed that the optimization issues of file   compression were too complex to try to automatically have browsers   decide that files should be compressed.  Many link-layer transport   mechanisms (e.g., high-speed modems) perform data compression over   the link, and optimizing for compression at this layer might not be   appropriate. It might be possible for browsers to optionally produce   a content-transfer-encoding of x-compress for file data, and for   servers to decompress the data before processing, if desired; this   was left out of the proposal, however.   Similarly, the proposal does not contain a mechanism for encryption   of the data; this should be handled by whatever other mechanisms are   in place for secure transmission of data, whether via secure HTTP or   mail.5.2 Deferred file transmission   In some situations, it might be advisable to have the server validate   various elements of the form data (user name, account, etc.)  before   actually preparing to receive the data.  However, after some   consideration, it seemed best to require that servers that wish to do   this should implement this as a series of forms, where some of the   data elements that were previously validated might be sent back to   the client as 'hidden' fields, or by arranging the form so that the   elements that need validation occur first.  This puts the onus of   maintaining the state of a transaction only on those servers that   wish to build a complex application, while allowing those cases that   have simple input needs to be built simply.   The HTTP protocol may require a content-length for the overall   transmission. Even if it were not to do so, HTTP clients are   encouraged to supply content-length for overall file input so that a   busy server could detect if the proposed file data is too large to be   processed reasonably and just return an error code and close the   connection without waiting to process all of the incoming data.  Some   current implementations of CGI require a content-length in all POST   transactions.   If the INPUT tag includes the attribute MAXLENGTH, the user agent   should consider its value to represent the maximum Content-Length (inNebel & Masinter              Experimental                      [Page 6]

RFC 1867             Form-based File Upload in HTML        November 1995   bytes) which the server will accept for transferred files.  In this   way, servers can hint to the client how much space they have   available for a file upload, before that upload takes place.  It is   important to note, however, that this is only a hint, and the actual   requirements of the server may change between form creation and file   submission.   In any case, a HTTP server may abort a file upload in the middle of   the transaction if the file being received is too large.5.3 Other choices for return transmission of binary data   Various people have suggested using new mime top-level type   "aggregate", e.g., aggregate/mixed or a content-transfer-encoding of   "packet" to express indeterminate-length binary data, rather than   relying on the multipart-style boundaries.  While we are not opposed   to doing so, this would require additional design and standardization   work to get acceptance of "aggregate".  On the other hand, the   'multipart' mechanisms are well established, simple to implement on   both the sending client and receiving server, and as efficient as   other methods of dealing with multiple combinations of binary data.5.4 Not overloading <INPUT>:   Various people have wondered about the advisability of overloading   'INPUT' for this function, rather than merely providing a different   type of FORM element.  Among other considerations, the migration   strategy which is allowed when using <INPUT> is important.  In   addition, the <INPUT> field *is* already overloaded to contain most   kinds of data input; rather than creating multiple kinds of <INPUT>   tags, it seems most reasonable to enhance <INPUT>.  The 'type' of   INPUT is not the content-type of what is returned, but rather the   'widget-type'; i.e., it identifies the interaction style with the   user.  The description here is carefully written to allow <INPUT   TYPE=FILE> to work for text browsers or audio-markup.5.5 Default content-type of field data   Many input fields in HTML are to be typed in. There has been some   ambiguity as to how form data should be transmitted back to servers.   Making the content-type of <INPUT> fields be text/plain clearly   disambiguates that the client should properly encode the data before   sending it back to the server with CRLFs.5.6 Allow form ACTION to be "mailto:"   Independent of this proposal, it would be very useful for HTML   interpreting user agents to allow a ACTION in a form to be aNebel & Masinter              Experimental                      [Page 7]

RFC 1867             Form-based File Upload in HTML        November 1995   "mailto:" URL. This seems like a good idea, with or without this   proposal. Similarly, the ACTION for a HTML form which is received via   mail should probably default to the "reply-to:" of the message.   These two proposals would allow HTML forms to be served via HTTP   servers but sent back via mail, or, alternatively, allow HTML forms   to be sent by mail, filled out by HTML-aware mail recipients, and the   results mailed back.5.7 Remote files with third-party transfer   In some scenarios, the user operating the client software might want   to specify a URL for remote data rather than a local file. In this   case, is there a way to allow the browser to send to the client a   pointer to the external data rather than the entire contents? This   capability could be implemented, for example, by having the client   send to the server data of type "message/external-body" with   "access-type" set to, say, "uri", and the URL of the remote data in   the body of the message.5.8 File transfer with ENCTYPE=x-www-form-urlencoded   If a form contains <INPUT TYPE=file> elements but does not contain an   ENCTYPE in the enclosing <FORM>, the behavior is not specified.  It   is probably inappropriate to attempt to URN-encode large quantities   of data to servers that don't expect it.5.9 CRLF used as line separator   As with all MIME transmissions, CRLF is used as the separator for   lines in a POST of the data in multipart/form-data.5.10 Relationship to multipart/related   The MIMESGML group is proposing a new type called multipart/related.   While it contains similar features to multipart/form-data, the use   and application of form-data is different enough that form-data is   being described separately.   It might be possible at some point to encode the result of HTML forms   (including files) in a multipart/related body part; this is not   incompatible with this proposal.5.11 Non-ASCII field names   Note that mime headers are generally required to consist only of 7-   bit data in the US-ASCII character set. Hence field names should be   encoded according to the prescriptions ofRFC 1522 if they contain   characters outside of that set. In HTML 2.0, the default characterNebel & Masinter              Experimental                      [Page 8]

RFC 1867             Form-based File Upload in HTML        November 1995   set is ISO-8859-1, but non-ASCII characters in field names should be   encoded.6. Examples   Suppose the server supplies the following HTML:     <FORM ACTION="http://server.dom/cgi/handle"           ENCTYPE="multipart/form-data"           METHOD=POST>     What is your name? <INPUT TYPE=TEXT NAME=submitter>     What files are you sending? <INPUT TYPE=FILE NAME=pics>     </FORM>   and the user types "Joe Blow" in the name field, and selects a text   file "file1.txt" for the answer to 'What files are you sending?'   The client might send back the following data:        Content-type: multipart/form-data, boundary=AaB03x        --AaB03x        content-disposition: form-data; name="field1"        Joe Blow        --AaB03x        content-disposition: form-data; name="pics"; filename="file1.txt"        Content-Type: text/plain         ... contents of file1.txt ...        --AaB03x--   If the user also indicated an image file "file2.gif" for the answer   to 'What files are you sending?', the client might client might send   back the following data:        Content-type: multipart/form-data, boundary=AaB03x        --AaB03x        content-disposition: form-data; name="field1"        Joe Blow        --AaB03x        content-disposition: form-data; name="pics"        Content-type: multipart/mixed, boundary=BbC04y        --BbC04y        Content-disposition: attachment; filename="file1.txt"Nebel & Masinter              Experimental                      [Page 9]

RFC 1867             Form-based File Upload in HTML        November 1995        Content-Type: text/plain        ... contents of file1.txt ...        --BbC04y        Content-disposition: attachment; filename="file2.gif"        Content-type: image/gif        Content-Transfer-Encoding: binary          ...contents of file2.gif...        --BbC04y--        --AaB03x--7. Registration of multipart/form-data   The media-type multipart/form-data follows the rules of all multipart   MIME data streams as outlined inRFC 1521. It is intended for use in   returning the data that comes about from filling out a form. In a   form (in HTML, although other applications may also use forms), there   are a series of fields to be supplied by the user who fills out the   form. Each field has a name. Within a given form, the names are   unique.   multipart/form-data contains a series of parts. Each part is expected   to contain a content-disposition header where the value is "form-   data" and a name attribute specifies the field name within the form,   e.g., 'content-disposition: form-data; name="xxxxx"', where xxxxx is   the field name corresponding to that field. Field names originally in   non-ASCII character sets may be encoded using the method outlined inRFC 1522.   As with all multipart MIME types, each part has an optional Content-   Type which defaults to text/plain.  If the contents of a file are   returned via filling out a form, then the file input is identified as   application/octet-stream or the appropriate media type, if known.  If   multiple files are to be returned as the result of a single form   entry, they can be returned as multipart/mixed embedded within the   multipart/form-data.   Each part may be encoded and the "content-transfer-encoding" header   supplied if the value of that part does not conform to the default   encoding.   File inputs may also identify the file name. The file name may be   described using the 'filename' parameter of the "content-disposition"   header. This is not required, but is strongly recommended in any case   where the original filename is known. This is useful or necessary in   many applications.Nebel & Masinter              Experimental                     [Page 10]

RFC 1867             Form-based File Upload in HTML        November 19958. Security Considerations   It is important that a user agent not send any file that the user has   not explicitly asked to be sent. Thus, HTML interpreting agents are   expected to confirm any default file names that might be suggested   with <INPUT TYPE=file VALUE="yyyy">.  Never have any hidden fields be   able to specify any file.   This proposal does not contain a mechanism for encryption of the   data; this should be handled by whatever other mechanisms are in   place for secure transmission of data, whether via secure HTTP, or by   security provided by MOSS (described inRFC 1848).   Once the file is uploaded, it is up to the receiver to process and   store the file appropriately.9.  Conclusion   The suggested implementation gives the client a lot of flexibility in   the number and types of files it can send to the server, it gives the   server control of the decision to accept the files, and it gives   servers a chance to interact with browsers which do not support INPUT   TYPE "file".   The change to the HTML DTD is very simple, but very powerful.  It   enables a much greater variety of services to be implemented via the   World-Wide Web than is currently possible due to the lack of a file   submission facility.  This would be an extremely valuable addition to   the capabilities of the World-Wide Web.Nebel & Masinter              Experimental                     [Page 11]

RFC 1867             Form-based File Upload in HTML        November 1995Authors' Addresses   Larry Masinter   Xerox Palo Alto Research Center   3333 Coyote Hill Road   Palo Alto, CA 94304   Phone:  (415) 812-4365   Fax:    (415) 812-4333   EMail:   masinter@parc.xerox.com   Ernesto Nebel   XSoft, Xerox Corporation   10875 Rancho Bernardo Road, Suite 200   San Diego, CA 92127-2116   Phone:  (619) 676-7817   Fax:    (619) 676-7865   EMail:   nebel@xsoft.sd.xerox.comNebel & Masinter              Experimental                     [Page 12]

RFC 1867             Form-based File Upload in HTML        November 1995A. Media type registration for multipart/form-dataMedia Type name: multipartMedia subtype name: form-dataRequired parameters: noneOptional parameters: noneEncoding considerations: No additional considerations other than as for other multipart types.Published specification:RFC 1867Security Considerations  The multipart/form-data type introduces no new security  considerations beyond what might occur with any of the enclosed  parts.References[RFC 1521] MIME (Multipurpose Internet Mail Extensions) Part One:           Mechanisms for Specifying and Describing the Format of           Internet Message Bodies.  N. Borenstein & N. Freed.           September 1993.[RFC 1522] MIME (Multipurpose Internet Mail Extensions) Part Two:           Message Header Extensions for Non-ASCII Text. K. Moore.           September 1993.[RFC 1806] Communicating Presentation Information in Internet           Messages: The Content-Disposition Header. R. Troost & S.           Dorner, June 1995.Nebel & Masinter              Experimental                     [Page 13]