There is only one file type that uses the .NSL extension:
An NSL file is a text file written in nsLanguage (nsL), a high-level language used to write Nullsoft Scriptable Install System (NSIS) installation wizards. It contains nsL code used to write and build NSIS installers.
NSIS (Nullsoft Scriptable Install System) script is a language used for declaring the logic and tasks for Windows software installers. The nsL Assembler for NSIS (with NSIS installed) is used to translate the NSL file into .NSI script for compiling into software installers.
Updated: July 22, 2016