Not working here on Opensuse 42.3 with Firefox and Chromium. A left click is displaying the midi as html (or gobbledegook).

As a comparison, the midis on your site are working as I might hope. Firefox is giving me the "open" options of Rosegarden and vlc and Chromium is opening them with vlc

An obvious difference is that the mudcat files use the .mid extension and yours use .midi and there may be something in my setup there - I'm not sure - maybe the server mime types are wrong? - I really can not remember how it works.