From Final Fantasy Inside
Data File Format
Located in the <installation folder>\Data\ folder, there are five (or six for the maximum installation) sets of three files.
.fs (File Source)
This is the main data source for all the files in a given category. The files are either LZSed or left raw (See notes on .fi files) and all concatenated together.
.fl (File List)
This is a simple flatfile using a DOS-style text encoding (uses both Carriage_Return and Line_Feed to indicate a new line) containing a sequential list of absolute paths and file names of the files stored in the .fs file.
.fi (File Index)
For each file in the .fs archive, there are 3 Long Words (12 bytes) of data in this file.
|Length of unpacked file||0x00|
|Location of packed file with .fs archive||0x04|
|Compression used (1 = LZS; 0 = None)||0x08|