I think the fix would be to pass in the adjusted base_url to FileSystemStorage in the django_bfm.utils.Directory class and then you could use FileSystemStorage.url() instead of "file['url'] = settings.MEDIA_URL + key" in the collect_metadata() method.
An "upload from URL" feature would be nice, where you could paste the (http) URL of the file and BFM would download it and put it among other files. That would save the trouble of downloading the file to user's own computer (especially if it's a big file).