Is it .DMS? A DMS file is a compressed disk, kind of like an ADF file. If you grab XDMS from the net and shove it in your path in windoze then you can select it as a disk in WinUAE (don't know about other versions). Or you could download a tiny program for the real Amiga to put it on real disks. That's all assuming it is DMS :-D
