In the Save As dialog you can select from 4 different .mid file formats.
 
So which one do you get when dragging a MIDI clip to the desktop? Is there even a way to know and can it be changed?

It doesn't matter if it is a single channel midi clip. It only matters if you need multi channel midi to load as separate tracks then you use Midi 1. Midi 0 will save as a combined track with all channels in one.
Far as I can tell dragging a midi track to the desktop is midi 0 because it a single clip. Dragging multiple clips will create multiple desktop icons. 
open in Sonar to test.