There is only one file type that uses the .NES extension:
Nintendo (NES) ROM File
An NES file is a game ROM created from an NES (Nintendo Entertainment System) video game. It contains the same data as the original NES cartridge and can be opened and played on a Mac or PC using an NES emulator.
NES files are used by gamers who enjoy playing retro, discontinued games on their computers. The gamers use emulators to play the NES games, including Nestopia, MESS, and FCEUX.
Some NES emulators play certain ROM files better than others. If you notice a game's audio or video performance is choppy in one emulator, you may want to try another emulation program.
Updated: October 29, 2019