like d4ni mentioned. if you already have the locate plugin, create a new alias with the following details.
Group Alias Name: Playlist
RegEx Pattern: ^playlist (.*)
Result(s): dosearch loc -txspf $$1
then type 'Playlist' in Farr and all the files with that extension would appear.
note: i did something like that before
but now with the availability of locate plugin, the above method is better.