Just type in the extension in the search tab. eg: .mp3
For audio, put the extension in the Title search box of the Audio type search. Likewise use specific media type search for other extensions. eg: Image type search .jpg, etc.

Characters such as * etc are not recognised by LW's search engine.

BTW you can filter your search results too using this technique explained here http://www.gnutellaforums.com/limewi...filtering.html