title: Symfony 2 form field - image preview instead of file path
body: <p>I write an application with Symfony 2. I have a form which has a file field.<br>
  When there is no file chosen the view is a button of "choose file" and on the right of it text 'No file chosen'. When i select a file, the file-name appears on the right of the button.</p>
  
  <p>I would like to change this behaviour:<br>
  When no file is chosen, show <strong>only</strong> the button.<br>
  After choosing a file, instead of showing the file-name I would like to generate a preview of the image.  </p>
  
  <p>Anyone has a simple solution for this problem?<br>
  Thanks!</p>