I downloaded django 1.6.5 and tried to migrate an old application (from Django 1.3).
Then when i go into the admin i got an error (see the image error2.jpg). As you can see, there is impossible to figure out what can be the reason of the problem.
Then I tried to replicate in my old environment I finally i replicate it (see the image error1.jpg). It helped me to see that the problem was because my MEDIA_ROOT doesn't have "/" at the end.
So I added the "/" on my updated application and it worked,
I think this is an error of django 1.6, not sure if it occurs on 1.5 but i open this discussion for developers.