On OSX the options are in the Preferences (edit extensions to share/show in Sharing->Shared Extensions). I recall a while ago having to manually add ".toast" there in order to search for and share those kinds of images. I think that list no longer prevents the display of results, since I was able to search for ".torrent" extensions recently.

You can also compile LW (opensource) and modify the source if that is more to your liking/abilities http://www.limewire.org/gettingstarted.shtml

btw--Jens-Uwe Mager compiles an OSX version that also has support for BeanShell http://www.mager.org/LimeWire/LimeWire.html