title: How to add additional non-entity fields to entity form in Symfony2
body: <p>I have created form with one element from Entity:</p>
  
  <pre><code>$promo = new Promo();
  
  $form = $this-&gt;createFormBuilder($promo)
          -&gt;add('code', 'text')
          -&gt;getForm();
  </code></pre>
  
  <p>And I want to add file element (this field doesn't exist in the Entity). When I do:</p>
  
  <pre><code>$form = $this-&gt;createFormBuilder($promo)
          -&gt;add('code', 'text')
          -&gt;add('image', 'file')
          -&gt;getForm();
  </code></pre>
  
  <p>I have an error: Neither property "image" nor method "getImage()". How can I add this field?</p>