Hmmm, sort of... If it's a MOD file, it's not a simple sound file. It's more like a MIDI file with the instrument sounds embedded in it. If you have a MOD player on your system, try opening it with that.
The "Unknown datatype" error is because the system doesn't have a matching datatype for that file format. If you look in the drawer Workbench:Devs/Datatypes you'll see a list of datatypes. Basically, if you get a datatype for a particular file format and install it, the Amiga can then understand that format, for example JPEG and Wave datatypes for opening .jpg images and .wav sound samples. However, MOD files are a little more complicated and work best in a dedicated player. You can find plenty on AmiNet:
http://aminet.net/mus/play
