NSIS Script File
NSIS is a install system that uses scripts that is developed by Nullsoft, the programs aim is to be stable whilst not needing very much space and to be flexible allowing the the user to create a installer that can install a program and all its components.
The NSIS system uses files that have the .nsi file extension, these are script files used by the program.
How to open .nsi files?
Software to open .nsi files
What are .nsi files