don't crash if a mediafile object is created without an existing file
Can no longer rely upon user.username with the advent of custom user models.
ItemEditor.change_view permission check in f7c0d77c overlooked object-level.
Merge remote-tracking branch 'DrMeers/next' into next

* DrMeers/next:
  ItemEditor.change_view permission check in f7c0d77c overlooked object-level.
  Can no longer rely upon user.username with the advent of custom user models.