There are 4 indexes;
0 Archive - Stores additional data and configs; sprites, textures, fonts, item definitions, map indices etc...
1 Models - Model .dat files in .gzip
2 Animations - .dat in .gzip
3 Music - midi in .gzip
4 Maps - .dat in .gzip
.dat's are just a generic file ending, no specific format. Look at the loaders in a
refactored client to see how each one loads and stores data.
Also worth looking at the code behind cache editors such as
rsam gui
Also a quick google turned up
this little site of info