There is only one file type that uses the .NSS extension:
Magentic Screensaver File
Screensaver file used by Magentic, a free desktop background and screensaver program; includes images and animations that run when the computer has been idle for a set amount of time; may be included with the Magentic software or downloaded from the Internet.
IncrediMail Magentic is no longer developed.
Updated: September 15, 2020