There is only one file type that uses the .NS1 extension:
Nestopia Save State
Save state created and used by Nestopia, a Nintendo Entertainment System (NES) emulator; stores a saved game state that allows you to resume your game from the save point point at a later time.
Nestopia provides 9 quick save slots that use the ".ns1" to ".ns9" extensions for saving states of a game. The save state will have the same name as the .NES ROM file it is created from. If the NES file is named game.nes then the NS1 file is named game.ns1.
Updated: June 15, 2015