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.
For information on using patterns in The GIMP, see the Pattern Selection dialog help page or the Bucket Fill tool help page.