Deprecated: implement this data type properly
There are three forms of System Exclusive Messages in MIDI files: monolithic, chopped into packets, escape form (with unrestricted binary data).
Currently we only support first and last type explicitly. But we leave the trailing 0xF7 markers which can be used to detect whether the messages are actually meant as packets.
Since I don't know where manufacturer information is in the packets form, I omit manufacturer handling for now.