Playlist file type for Express

What are the file types or file extensions used on a Playlist for the Sansa Express?  Will more than one kind of file type work?  Are .ZPL or .PLA valid Express Playlist file types?

What types of software, other than Windows Media Player, can be used to create the Express Playlists?  I don’t particularly like the Windows Media Player and would like to find an alternative Playlist creator for the Express.