http://filext.com/detaillist.php?extdetail=fat
fat = Zinf Audio Player Theme File
Zinf is based on what was called FreeAmp, an open-source player. However, AMP is a trademark of PlayMedia Systems, Inc., and therefore the original name of the project was changed to Zinf which translates to "Zinf Is Not FreeA*p!".
big = Electronic Arts Game MOD File
BIG files control various parts of Electronic Arts games. A utility called FinalBig (search for FINALBIG.EXE) is said to be able to create individual MODs which can then be imported into the various games. The games usually have command-line parameters you can use to play the MOD if you want to keep your game files intact.
http://filext.com/detaillist.php?extdetail=big