.ass
From Halopedia, the Halo wiki
.ass or amalgam scene specification is a source file type. An intermediary format file produced by the 3ds Max exporter. When run through Halo 2 Tool, .ass files generate .scenario and .scenario_structure_bsp files, which can then be edited by other tools.[1] When Halo 2 (Windows Vista) was initially released, an ".ass error" would occur, showing a picture of Bungie employee Charlie "Chucky" Goughs uncovered backside.[2] This has since been patched.
Trivia
If you pay attention to the picture, it has 7 warnings, a reference to Bungie's favorite number.
