This should go into2.3 as this is bugfix.

I also once noticed this problem. But

In the International System of Quantities, the kilobyte (symbol kB) is 1000 bytes. The binary representation of 1024 bytes typically uses the symbol KB, using an upper-case K. Informally sometimes the B is dropped, making K generally understood as 1024 bytes; however, this usage is not standardized and may be found used arbitrarily.
http://en.wikipedia.org/wiki/Kilobyte

So since we usekB in the validation message one can argue it's correct at the moment.

@Tobion all systems are using base 2 for their file sizes (although HDD are marketed using base 10 to have bigger numbers), so most people are expecting base2 when seeing file sizes as they are accustomated to it (even without knowing it is base2)

Sure but we need to change the unit symbol then.

@snoob Can you finish this one by applying the suggestion made by@Tobion? Thanks.

@fabpot Done

There are 2 "kB" suffixes remaining in the classes Symfony\Component\HttpFoundation\File\UploadedFile and Symfony\Component\Console\Helper\Helper. The size seems to be on base 2 so maybe we should change these suffixes there too.

@snoob Thanks, can you change the 2 other ones so that we are consistent everywhere in the framework (you can do so in this PR)?

I think we need to change even more and use the binary prefixes of IEC.
But since currently $constraint->maxSize reads SI prefixes (M andk), it should also validate against SI prefixes as it has been. If we want to support base 2, then the constraint should also allow binary prefixes (Ki,Mi) and validate them accordingly. This way developers can choose which style they prefer and it avoids BC breaks.
If we change the validation as done at the moment, it just adds to the confusion.

Accordingly, the SI prefixes should only be used in the decimal sense, even when referring to data storage capacities: kilobyte and megabyte denote one thousand bytes and one million bytes respectively (consistent with SI), while new terms such as kibibyte, mebibyte and gibibyte, having the symbols KiB, MiB, and GiB, denote 1024bytes, 1048576bytes, and 1073741824bytes, respectively.
http://en.wikipedia.org/wiki/Binary_prefixes

@fabpot : Done

Definitely -1 on the current state. Prefer to go my above suggestion

@Tobion : I agree with the IEC convention but there is a problem in my opinion, it is not widespread at all.

Do we really want to keep the base 10 system ?

If we chose to do, we can't use KB, because in my opinion, the difference between kb and KB is too thin and it will create confusions and mistakes.

If we decide not to use the base 10 system, then I think it is better to use KB as it is clearer than Ki.

There are no BC breaks here. It was an expected behavior.

Continued in#10951.