gajim:// and .gaj
Protocol
Using protocol scheme gajim:// to install plugins with URI addresses, like adiumxtras.com do with adiumxtra://.
File Type
Using .gaj or .gap or .gpl for the .zip file archives (Gajim Plugins).
shared-mime-info
After some time we would need to ask to add .gaj extension to shared-mime-info database http://freedesktop.org/wiki/Software/shared-mime-info/