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" option

Information
-------------

Following this tutorial ["How to Upload Files"][1] but storing many `brochures` instead of one, i.e.:

```php
// src/AppBundle/Entity/Product.php

class 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 file

Information
-------------

Based on the same information before, but adding some constraints:

```php
// src/AppBundle/Entity/Product.php

use 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.php

use 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.php

use 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.html

Commits
-------

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 with https://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