... It's not so much a "PS" file extention.. more like a Playstation game.
I mean though, the PS has standard APIs for accessing from some sort of packages.... (like Windows has for accessing files)
So when I am referring to "PS File" that means, any file that is readable by the PS filesystem.... sort of like the Virtual File Systems employed by most newer games... (examples:
PAK, WAD - Half Life / Quake
VOL - Outpost2
MPQ, SCM, SCX, ... - Starcraft, Warcraft3, Diablo2+, most newer Blizzard games
WAR - Warcraft 1 and 2
MIX, MMX, MMP, ... - Westwood / Command & Conquer (newer versions, Tiberian sun, RA, and up)
DRS - Age of Empires 1 and 2
... you get the point)
they treat each of these files as a sort of "Hard drive in a file". I imagine it's the same way for the PS/PS2/Xbox/Gamecube/ etc.