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

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

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

 {% comment %}
  * Copyright (c) 2016 Idiap Research Institute, http://www.idiap.ch/
  * Contact: beat.support@idiap.ch
- * 
+ *
  * This file is part of the beat.web module of the BEAT platform.
- * 
+ *
  * Commercial License Usage
  * Licensees holding valid commercial BEAT licenses may use this file in
  * accordance with the terms contained in a written agreement between you
  * and Idiap. For further information contact tto@idiap.ch
- * 
+ *
  * Alternatively, this file may be used under the terms of the GNU Affero
  * Public License version 3 as published by the Free Software and appearing
  * in the file LICENSE.AGPL included in the packaging of this file.
  * The BEAT platform is distributed in the hope that it will be useful, but
  * WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
  * or FITNESS FOR A PARTICULAR PURPOSE.
- * 
+ *
  * You should have received a copy of the GNU Affero Public License along
  * with the BEAT platform. If not, see http://www.gnu.org/licenses/.
 {% endcomment %}
@@ -228,7 +228,7 @@
     {% endwith %}{# groups, uses or parameters #}
 
     {% if open_source %}
-    <textarea class="form-control" id="code-display">{{ object.source_code_file.read }}</textarea>
+    <textarea class="form-control" id="code-display">{{ object.source_code }}</textarea>
     <p class="help-block">{{ texts.code|safe }}</p>
     {% else %}
     <div class="alert alert-warning">