[dataformats][templates] Use file field property to read content

Dataformats have properties related to their file fields
that does the proper handling for reading their content
use that.

beat/web/dataformats/templates/dataformats/panels/editor.html

@@ -43,7 +43,7 @@
         </div>
       </div>
       {% endif %}
-      <textarea class="form-control" id="declaration-display">{{ object.declaration_file.read }}</textarea>
+      <textarea class="form-control" id="declaration-display">{{ object.declaration }}</textarea>
       <p class="help-block">{{ texts.json|safe }}</p>
     </div>
   </div>