There is only one file type that uses the .VMF_AUTOSAVE extension:
Valve Hammer Editor Autosave File
Auto-generated file created by Hammer Editor, a program used to build maps for Valve games such as Half-Life; stores an autosaved, backup copy of the currently opened .VMF file.
VMF_AUTOSAVE files are used for recovering map data in the event the VMF file becomes corrupted, or if the program unexpectedly crashes.
|Valve Hammer Editor||Free|
Updated: June 17, 2010