Hey, Variorum. I've been scouring the net looking for info on this midi but found very little so far.
What I've found suggests that it's not a WinJammer demo file, but rather the "WinJammer Demo" refers to the shareware WinJammer Demo (as opposed to WinJammer Pro), which puts WinJammer Demo in every midi's meta info. I believe this as I've found many midis with "WinJammer Demo" in the midi info. It probably served as a watermark of sorts back in the day.
I do have an idea to find out what the midi might be though, but I would need the midi to do so. I'm a midi connoisseur and I have dumped several Gb of midis from the web onto a hard drive. If I had the midi I could compare byte for byte the content of the midi with all the midis in my collection, potentially I might already have the midi under another name besides "SONG5.MID" as many duplicates of the same midi exist on the web under many differing names. I also just want the midi because it's an awesome piece and I'd love to mess around with it myself.
Let me know what you think until then I'll continue my search for info on this midi.