[fix bug 1082208] Support media files and CDN.

In my local testing this doesn't actually keep the old filename while saving.

Personally I'm a fan of using a simple `OverwritingStorage` class: http://stackoverflow.com/questions/9522759/imagefield-overwrite-image-file-with-same-name

(Especially because you just use self.exists and self.delete, keeps it really simple)