.FAT File

File extension fat is most likely best known for its in FreeAmp and later also its successor program Zinf, which are freeware media players for Windows and Linux.

A .fat files contains skin / theme that can be used to customize the look of the program.

Files with fat suffix are skins or themes for either old FreeAmp or newer Zinf media player.

How to open:

You can use .fat files when you change the ui look of the program.

How to convert:

A conversion of any kind for .fat files is unlikely, but they appear to be ZIPs that can be extracted with proper tool.