Seems to be a little bug in MameUI. Setting more than a single path in swpath at machine.ini, when you want to open a file the explorer opens the snap directory. For example in cpc6128.ini:
-Media view, Floppy disk1,Mount file:
swpath Z:\Juegos\Amstrad\Disks --> Works great, opens the Disk folder
swpath Z:\Juegos\Amstrad\Disks; Z:\Juegos\Amstrad\Tapes --> Opens the snap folder
Happened in .0217 as well.