There is only one file type that uses the .OFF extension:
Object File Format
ASCII based format used for describing 3D objects; defines collections of planar polygons and vertices that create the object's surface.
Each OFF file begins with the keyword "OFF." The next line includes the number of vertices, faces, and edges of the model. Vertices and faces are listed with x, y, z coordinates, one per line.
Updated: January 18, 2016