The PAT file format is used for GIMP patterns. When you save a pattern file, you will be prompted to give a description. This is not the filename, but an internal description which gets displayed at the top of the Patterns Selection dialog when loaded.
To create a pattern to be used by The GIMP from the Pattern Selection dialog, you must save it in the your /patterns directory inside your personal GIMP directory — normally ~/.gimp-1.2. Make sure you press Refresh in the Pattern Selection dialog to rescan the patterns directory.