GUEST wrote: I'd guess then that there is a mime type issue on the mudcat server.

Yes.

This one works - from GUEST's link:
chopin-nocturne-op9-no2.mid
Content-Type: audio/x-midi

This from Mudcat:
WayToBeHappy-HC.mid
Content-Type: audio/mid,text/html; charset=utf-8

I've never seen two mime-types on a file before - I'm not sure if it's valid but it's certainly wrong here. Looks like my browser (Firefox 60 OpenSuSE Leap 15) accepts the second for some reason and displays text. But I'm not surprised if other browsers behave differently.