title: Empty $request->files (FileBag) when using a custom form (not the built in Type from SF)
body: <p>I'm having issues with file uploads... I'm adding manually a few inputs within a Symfony form (Offer) to allow the user to create a new Company if he doesn't want to use one that already exists, it works very well for the text/textarea fields, but my file Input doesn't work properly. Its content appears in the request parameters instead of in the files parameters of my Request, see below what I get when I dump($request) :</p>
  
  <pre><code> OfferController.php on line 173:
  Request {#86 ▼
    +attributes: ParameterBag {#70 ▶}
    +request: ParameterBag {#69 ▼
      #parameters: array:2 [▼
        "offer" =&gt; array:15 [▶]
        "company" =&gt; array:6 [▼
          "name" =&gt; "entreprise"
          "pic_logo" =&gt; "ah.png" //THIS SHOULD NOT BE HERE BUT IN THE FILEBAG
          "sector" =&gt; "2"
          "status" =&gt; "fdsfds"
          "revenues" =&gt; "sdfsdf"
          "content" =&gt; "&lt;p&gt;sdfdsf&lt;/p&gt;\r\n"
        ]
      ]
    }
    +query: ParameterBag {#49 ▶}
    +server: ServerBag {#73 ▶}
    +files: FileBag {#63 ▼
      #parameters: [] //NO FILE HERE :'(
    }
  </code></pre>
  
  <p>Here is the code that produces this :
  View :</p>
  
  <pre><code>{{ form_start(form) }}
  
      &lt;h2&gt;Employeur&lt;/h2&gt;
      {{ form_row(form.company) }}
  
      &lt;a class="company-new" href="#" id="toggleCompanyForm"&gt;Ajouter une nouvelle entreprise...&lt;/a&gt;
  
      &lt;div id="form_company" style="display:none"&gt;
          &lt;div class="row"&gt;
              &lt;div class="large-6 small-12"&gt;
                  &lt;label&gt;Nom de l'entreprise&lt;/label&gt;
                  &lt;input type="text" name="company[name]"&gt;
              &lt;/div&gt;
              &lt;div class="large-6 small-12"&gt;
                  &lt;label&gt;Logo de l'entreprise&lt;/label&gt;
                  {% for message in app.session.flashBag.get('warning_logo') %}
                      &lt;div class="alert alert-warning"&gt;
                          {{ message }}
                      &lt;/div&gt;
                  {% endfor %}
                  &lt;input type="file" name="company[pic_logo]"&gt; {# MY FILE INPUT #}
              &lt;/div&gt;
  </code></pre>
  
  <p>And the controller :</p>
  
  <pre><code>public function createAction(Request $request)
  {
      $confirmed = false;
      $entity = new Offer();
  
      $em = $this-&gt;getDoctrine()-&gt;getManager();
      $sectors = $em-&gt;getRepository('ModelBundle:Sector')-&gt;findAll();
  
      $form = $this-&gt;createForm('ModelBundle\Form\OfferType', $entity);
      $session    = $request-&gt;getSession();
  </code></pre>
  
  <p>die(dump($request));</p>