[1.0.x] Fixed #9508 -- Added an appropriate FileField.__hash__ implementation.

Required because we declare a custom __eq__ method.

Backport of r9997 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.0.X@10000 bcc190cf-cafb-0310-a4f2-bffc1f526a37