All H.264 WebRTC streams are recorded in mp4
All VP8 WebRTC streams are recorded in webM
So you can set VP8 as priority video codec:
codecs=opus,alaw,ulaw,g729,speex16,g722,mpeg4-generic,telephone-event,vp8,h264,flv,mpv
It should record webM
Or you can set H.264 as priority video codec...