Just info, for those wondering why you cannot save your file as unl in notepad.

Don't forget to change your folder setting to un-click the
"Hide extensions for known file types"

1.go to control panel
2.Folder Option
3.View
4.Advance settings
5.Un-tick the "Hide extensions for known file types"

You are done, then you can change the file to GMAPSUPP.UNL no more GMAPSUPP.UNL.TXT

Good Luck