There is only one file type that uses the .NIF extension:
Gamebryo Model File
3D model used by Gamebryo, a game engine used for Fallout 3, Warhammer Online: Age of Reckoning, and other popular video games; supports 3D meshes (geometry), textures, shading, and other model properties; used for storing terrain, game objects, or other 3D models.
NIF files are also used to store "rigged" models, which contain default geometries that are used as templates for creating new customized models. NIF files can be compiled by the Gamebryo LightSpeed game development software.
NOTE: NIF stands for "NetImmerse File." It is named after NetImmerse, the predecessor to Gamebryo.
Updated: September 7, 2010