media_types/__init__.py:109:
    if filename.find('.') > 0:

should be: if filename.find('.') >= 0:

Because otherwise files of the form "./blah.mp4" don't work, which happens with the localfiles plugin.

If I attempt to create a user that exists already by executing this command twice:

gmg adduser --username foo --password bar --email foo@bar.com

I get an error message at the second time: "Sorry, a user with that name already exists." but the exit code stays 0. This should be non-0, so it's easier to script this.

It may be worthwhile to check the exit codes of other gmg subcommands as well.

The front page of https://issues.mediagoblin.org/ says

"In the navigation bar in the upper right is a link titled Register"

but this does not seem to be correct. In other words, you get this bug report filed anonymously :-)