This PR was squashed before being merged into the 2.7 branch (closes#20418).Discussion----------[Form][DX] FileType "multiple" fixes| Q             | A| ------------- | ---| Branch?       | 2.7| Bug fix?      | yes| New feature?  | no| BC breaks?    | no| Deprecations? | no| Tests pass?   | yes| Fixed tickets |#12547| License       | MIT| Doc PR        | -# (1st) Derive "data_class" option from passed "multiple" optionInformation-------------Following this tutorial ["How to Upload Files"][1] but storing many `brochures` instead of one, i.e.:```php// src/AppBundle/Entity/Product.phpclass Product{    /**     *@var string[]     *     *@Orm\Column(type="array")     */    private $brochures;    //...}``````php//src/AppBundle/Form/ProductType.php$builder->add('brochures', FileType::class, array(    'label' => 'Brochures (PDF files)',    'multiple' => true,));```The Problem--------------I found a pain point here when the form is loaded again after save some brochures (Exception):> The form's view data is expected to be an instance of class Symfony\Component\HttpFoundation\File\File, but is a(n) array. You can avoid this error by setting the "data_class" option to null or by adding a view transformer that transforms a(n) array to an instance of Symfony\Component\HttpFoundation\File\File.The message is very clear, but counter-intuitive in this case, because the form field (`FileType`) was configured with `multiple = true`, so IMHO it shouldn't expect a `File` instance but an array of them at all events.The PR's effect---------------**Before:**```php$form = $this->createFormBuilder($product)    ->add('brochures', FileType::class, [        'multiple' => true,'data_class' => null, // <---- mandatory    ])    ->getForm();```**After:**```php$form = $this->createFormBuilder($product)    ->add('brochures', FileType::class, [        'multiple' => true,    ])    ->getForm();```# (2nd) Return empty `array()` at submit no fileInformation-------------Based on the same information before, but adding some constraints:```php// src/AppBundle/Entity/Product.phpuse Symfony\Component\Validator\Constraints as Assert;class Product{    /**     *@var string[]     *     *@Orm\Column(type="array")     *     *@Assert\Count(min="1") // or@Assert\NotBlank()     *@Assert\All({     *@Assert\File(mimeTypes = {"application/pdf", "application/x-pdf"})     * })     *     */    private $brochures;}```This should require at least one file to be stored.The Problem--------------But, when no file is uploaded at submit the form, it's valid completely. The submitted data for this field was `array(null)` so the constraints pass without any problem:* `@Assert\Count(min="1")` pass! because contains at least one element (No matter what)* `@Assert\NotBlank()` it could pass! because no `false` and no `empty()`* `@Assert\File()` pass! because the element is `null`Apart from that really we expecting an empty array instead.The PR's effect----------------**Before:**```php// src/AppBundle/Entity/Product.phpuse Symfony\Component\Validator\Constraints as Assert;class Product{    /**     *@var string[]     *     *@Orm\Column(type="array")     *     *@Assert\All({     *@Assert\NotBlank,     *@Assert\File(mimeTypes = {"application/pdf", "application/x-pdf"})     * })     *     */    private $brochures;}```**After:**```php// src/AppBundle/Entity/Product.phpuse Symfony\Component\Validator\Constraints as Assert;class Product{    /**     *@var string[]     *     *@Orm\Column(type="array")     *     *@Assert\Count(min="1") // or@Assert\NotBlank     *@Assert\All({     *@Assert\File(mimeTypes = {"application/pdf", "application/x-pdf"})     * })     *     */    private $brochures;}```  [1]:http://symfony.com/doc/current/controller/upload_file.htmlCommits-------36b7ba6 [Form][DX] FileType "multiple" fixes

…e (HeahDude)This PR was merged into the 2.7 branch.Discussion----------[Form] Fixed empty data on expanded ChoiceType and FileType| Q             | A| ------------- | ---| Branch?       | 2.7| Bug fix?      | yes| New feature?  | no| BC breaks?    | no| Deprecations? | no| Tests pass?   | yes| Fixed tickets |#25896| License       | MIT| Doc PR        | ~Alternative of#25924.I don't think we have to do this by adding overhead in master and getting it properly fixed in 2 years.This is bug I've introduced while fixing another bug#17822. Which then has been replicated in#20418 and#24993.I propose instead to clean up the code for all LTS, since this is a wrong behaviour that has never been documented, and most of all unreliable.The `empty_data` can by anything in the view format as long as the reverse view transformation supports it. Even an object derived from the data class could be invokable.I think we should remain consistent withhttps://github.com/symfony/symfony/blob/2.7/src/Symfony/Component/Form/Form.php#L615.Commits-------9722c06 [Form] Fixed empty data on expanded ChoiceType and FileType