There is only one file type that uses the .W3M extension:
Warcraft 3 Map File
Game map used by Warcraft III: Reign of Chaos; includes terrain, objects, and resources placed within the map; may be included with the game or created by players with the Warcraft III World Editor.
NOTE: Maps for the Warcraft III expansion pack use the .W3X extension.
Updated: August 1, 2010