[Ballbarian]

You should be able to select "Save As" from the "File" menu and change the three letter file extension from the dialog manually. The "Save As Type" field will still be text, but that's ok, it is still a text file, you just want to change the extension.

An alternative is to go ahead and save it with the txt extension, then rename the file manually by right clicking on the file and selecting "rename" from the pop up menu. Again, the goal is to change ".txt" to ".dm".