title: Symfony form: uploaded image is not shown in edit form
body: <p>I am trying to create form for Cafe entity.</p>
  
  <p>Cafe.php</p>
  
  <pre><code> class Cafe{
  
      /**
       * @var string $image
       * @Assert\File( maxSize = "1024k", mimeTypesMessage = "Please upload a valid Image")
       * @ORM\Column(name="image", type="string", length=255, nullable=true)
       */
       private $image;
   }
  </code></pre>
  
  <p>CafeType.php</p>
  
  <pre><code>/**
   * @param FormBuilderInterface $builder
   * @param array $options
   */
  public function buildForm(FormBuilderInterface $builder, array $options)
  {
      $builder
          -&gt;add('name', TextType::class, array('label' =&gt; 'Name'))
          -&gt;add('image', FileType::class, array(
              'label' =&gt; 'Image',
              'data_class' =&gt; null,
              'required'    =&gt; false
          ))
      ;
  }
  </code></pre>
  
  <p>CafeController.php</p>
  
  <pre><code>public function editAction(Request $request, Cafe $cafe)
  {
      $editForm = $this-&gt;createForm('AdminBundle\Form\CafeType', $cafe);
      $editForm-&gt;handleRequest($request);
  
      if ($editForm-&gt;isSubmitted() &amp;&amp; $editForm-&gt;isValid()) {
          $img = $editForm['image']-&gt;getData();
          if($img){
              $fileLocation = $this-&gt;get('app.s3_file_uploader')-&gt;putFileToBucket($img, 'cafe-images/'.uniqid().'/cafe-image');
              $cafe-&gt;setImage($fileLocation);
          }
  
          $em = $this-&gt;getDoctrine()-&gt;getManager();
          $em-&gt;persist($cafe);
          $em-&gt;flush();
  
          return $this-&gt;redirectToRoute('cafe_index');
      }
  
      return $this-&gt;render('AdminBundle:Cafe:edit.html.twig', array(
          'cafe' =&gt; $cafe,
          'edit_form' =&gt; $editForm-&gt;createView()
      ));
  }
  </code></pre>
  
  <p>In controller I am using custom service to save image in S3 bucket which returns image location-fileLocation. And then I set this file url to <code>$image</code> property of Cafe entity. </p>
  
  <p>Creating Cafe works fine. But when I try to edit cafe, upload input is empty. So that when I click save, old image is removed. I need to upload iamge everytime when I want to edit cafe. How can I keep image file in edit form?</p>