Fixed #21042 -- Allowed accessing FileDescriptor on the model class.

This is consistent with ability to reference other descriptors
on the model class (5ef0c03ae9).

django/db/models/fields/files.py

@@ -157,9 +157,7 @@ class FileDescriptor(object):
 
     def __get__(self, instance=None, owner=None):
         if instance is None:
-            raise AttributeError(
-                "The '%s' attribute can only be accessed from %s instances."
-                % (self.field.name, owner.__name__))
+            return self
 
         # This is slightly complicated, so worth an explanation.
         # instance.file`needs to ultimately return some instance of `File`,