improve mimetype handling
When trying
$ gdbus call --session --dest org.freedesktop.PackageKit --object-path /org/freedesktop/PackageKit --method org.freedesktop.PackageKit.Modify.InstallMimeTypes 0 "['application/vnd.flatpak.ref']" ""
on an rpm-ostree system, I get a "File format not found" error page, despite the mimetype being known to the system, as evidenced by:
$ gio mime application/vnd.flatpak.ref Default application for “application/vnd.flatpak.ref”: gnome-software-local-file.desktop
The error page is misleading in this case. You should do a local mimetype search to get a display name for the type anyway, which would reveal that the mimetype is actually known to the system.