We have been experimenting with using tgz backup ($CFG->enabletgzbackups) and even setting it as the default.

With the default changed, a few tests fail, and it exposed a problem with get_file_packer used in some contexts on backups without the correct mimetype argument. After auditing all such usage I came up with only one other place in core which seems incorrect.

Anyway, providing patch to get these places fixed up...