booru/site: Force jpg thumbnail extension on all Gelbooru sites
booru/site: Force jpg thumbnail extension on all Gelbooru sites

There seems to be a bug with some older versions of Gelbooru 0.2.0,
oddly enough Safebooru seems to have fixed the issue since I
implemented the fix specifically for them.

ahodesuka/ahoviewer,ahodesuka/ahoviewer,ahodesuka/ahoviewer