gthumb needs to include it's mime types in the .desktop for it to register with the new mime system.
Yeah, i sent a patch to the upstream maintainer of gthumb a few weeks back. Appears he committed it last week without either notifying me or crediting me. I'll add it to our package.
heh jok, I'll close this as UPSTREAM then.
er, "heh ok" is what i typed...
For the record, this will be fixed in gthumb-2.4.2-1 which I just built.