fix field clashes with in Django 1.9

Set cmsplugin_ptr and set related_name to avoid field name clashes.
see django-cms/django-cms#5030

Author: stefanfoulis

djangocms_file/migrations/0004_set_related_name_for_cmsplugin_ptr.py

@@ -0,0 +1,21 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.9.2 on 2016-03-04 04:10
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('djangocms_file', '0003_remove_related_name_for_cmsplugin_ptr'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='file',
+            name='cmsplugin_ptr',
+            field=models.OneToOneField(on_delete=django.db.models.deletion.CASCADE, parent_link=True, primary_key=True, related_name='djangocms_file_file', serialize=False, to='cms.CMSPlugin'),
+        ),
+    ]

djangocms_file/models.py

@@ -39,11 +39,12 @@ class File(CMSPlugin):
     The icon search is currently performed within get_icon_url; this is
     probably a performance concern.
     """
-    # Set cmsplugin_ptr and remove related_name to avoid field name clashes
-    # with any other plugin that has a field called "file".
+    # Add an app namespace to related_name to avoid field name clashes
+    # with any other plugins that have a field with the same name as the
+    # lowercase of the class name of this model.
     # https://github.com/divio/django-cms/issues/5030
     cmsplugin_ptr = models.OneToOneField(
-        CMSPlugin, related_name='+', parent_link=True)
+        CMSPlugin, related_name='djangocms_file_file', parent_link=True)
 
     file = models.FileField(_("file"), upload_to=get_plugin_media_path)
     title = models.CharField(