.bfstm files are the streamed music in Pikmin 3. Some of these songs contain several tracks, which are enabled or disabled in-game to make the music dynamic. This game's BFSTM files can be found in
/CMCmn/audio/audiores/stream. A rundown of the format can be found
here. A list of what files match what songs can be found here.
.bfstm files can be created using Looping Audio Converter.