Stems is a new audio format by Native Instruments which is targeted for DJs (http://www.stems-music.com/ ): basically, it's 4 MPEG AAC streams in a mp4 container. Unfortunately, since AcoustBrainz client doesn't support multi-channel files, it refuses to analyze it:

xplt@localhost:~/Blob/abzsubmit-0.1$ ./abzsubmit ~/Music/Under\ Break/

[... ] processing /home/xplt/Music/Under Break
[:( extract] /home/xplt/Music/Under Break/Power Fresh/01 Power Fresh.stem.mp4
Process step: Read metadata
Process step: Compute md5 audio hash and codec
AudioLoader ERROR: found 5 streams in the file, expecting only one audio stream

I think it'd be good, if such limitation was eliminated.