R1:9e5410e6a213

Rename `bin/files metadata` to `bin/files rebuild` and let it rebuild MIME information

Summary:
See <https://github.com/facebook/phabricator/issues/320>. Files can end up with a bad MIME type, and we don't update it when uploading another copy of the file since obviously the new copy has the same data and thus the same MIME type.

- Rename `bin/files metadata` to `bin/files rebuild` to make it a more consistent verb.
- Let it rebuild MIME types so users who hit issues like this can run `bin/files…