There is only one file type that uses the .NMF extension:
ArcGIS Explorer Map File
Map file created by ArcGIS Explorer, a free mapping program developed by ESRI; may store terrain, coordinates, points of interest, drawn shapes, 3D environment effects, and other map objects; can also store directions, routes, and measurements; used for saving and loading custom maps as well as sharing maps online.
NMF files can be used for viewing a map with the standalone ArcGIS Explorer application, or within a Web browser that has the ArcGIS Explorer plug-in.
Updated: March 10, 2010