.AMXX File Extension
What is an AMXX file?
Plug-in created for AMX Mod X, a metamod plugin for Half-Life and other similar games; designed for server administration; used for adding custom features to the game, such as statistics tracking and gameplay modifications.
Half-Life versions supported by AMX Mod X include Counter-Strike, Day of Defeat, Natural Selection, Team Fortress Classic, The Specialists, and Earth's Special Forces.
AMX Mod, the predecessor to AMX Mod X, saves plugins with the .AMX file extension.