I've got early FFVII playable demo. Some interesting info: there is no DAT and BSX file in there. =)
There are MIM file - the same as in usual game
BSC file - It seems it have the same function as BSX file. But it is bigger... a lot bigger =)
ATE file - This is first sector of DAT file.
CA file - camera info (4th sector of DAT)
ID file - walkmesh info (2nd sector in DAT)
MAP file - background info (3rd sector of DAT)
All files lzs compressed.
I wander what ID means. This name used in opcodes too... walkmesh - id... hmmm.
And I can't find triggers and encounter info. Don't know where they are.