title: Symfony3 Image validation for working
body: <p>If I don't validate what I am uploading things work fine but the moment I add validation to the image field I get an error of </p>
  
  <pre><code>Serialization of 'Symfony\Component\HttpFoundation\File\UploadedFile' is not allowed
  </code></pre>
  
  <p>The only thing at the moment I have inside my <code>validation.yml</code> is as following</p>
  
  <pre><code>AppBundle\Entity\User:
      properties:
          firstName:
              - NotBlank: {message: "user.firstname.not_blank"}
          lastName:
              - NotBlank: { message: "user.lastname.not_blank" }
          profileImage:
              - Image:
  </code></pre>
  
  <p>Inside my Entity</p>
  
  <pre><code>use Symfony\Component\HttpFoundation\File\File;
      public function setProfileImage(File $profileImage = null)
      {
          $this-&gt;profileImage = $profileImage;
  
          return $this;
      }
  </code></pre>
  
  <p>What am I missing here? This used to work fine in Symfony 2.7</p>