MP4.Payload.{AAC.AVC1}:new_track message to attach to some sink with pads in the :always availability mode (such as the RTMP sink) I can't attach that track to some grouping of elements which end at the sink temporarily. For example, if I get the :new_track notification for an AAC track I can't attach just the :audio pad of the RTMP sink, because when handling that callback there is no video pad to attach.ffprobe the mp4 file and parse the available tracks beforehand?