title: Validation in extended entity doesn`t work
body: <p>I have an Entity Media which have protected File value</p>
  
  <pre><code>&lt;?php
  
  namespace Chiave\MediaBundle\Entity;
  
  use Doctrine\ORM\Mapping as ORM;
  use Symfony\Component\Validator\Constraints as Assert;
  use Symfony\Component\HttpFoundation\File\UploadedFile;
  
  /**
   * @ORM\Table(name="media")
   * @ORM\Entity
   * @ORM\HasLifecycleCallbacks
   */
  class Media
  {
  
  
  ...
  
  /**
       * @Assert\File(
       *     maxSize="5M"
       * )
       */
      protected $file;
  </code></pre>
  
  <p>Then i extend this class in my AppBundle because I need only images to get pass validation</p>
  
  <pre><code>&lt;?php
  
  namespace AppBundle\Entity;
  
  use Symfony\Component\Validator\Constraints as Assert;
  use Doctrine\ORM\Mapping as ORM;
  
  /**
   * @ORM\Entity
   * @ORM\Table(name="image")
   */
  class Image extends \Chiave\MediaBundle\Entity\Media
  {
      /**
       * @Assert\Image()
       */
      protected $file;
  }
  </code></pre>
  
  <p>Also i have Form Type for this which looks like this</p>
  
  <pre><code>&lt;?php
  
  namespace AppBundle\Form;
  
  use Symfony\Component\Form\AbstractType;
  use Symfony\Component\Form\FormBuilderInterface;
  use Symfony\Component\OptionsResolver\OptionsResolverInterface;
  use AppBundle\Form\ImageType as ImageType;
  
  class POIType extends AbstractType
  {
  
      /**
       * @param FormBuilderInterface $builder
       * @param array                $options
       */
      public function buildForm(FormBuilderInterface $builder, array $options)
      {
          $builder
                  ...
                  -&gt;add('images', 'collection', [
                      'type' =&gt; new ImageType,
                      'allow_add' =&gt; true,
                      'allow_delete' =&gt; true,
                      'by_reference' =&gt; false,
                      'label' =&gt; ' ',
                  ]);
      }
  </code></pre>
  
  <p>My problem is that validation didn`t work. I can upload image yes, but i can also upload PHP file or... pretty much everything. Any ideas? I will be very grateful.</p>
  
  <p>Thanks in advance.</p>