There is only one file type that uses the .N2 extension:
Nitrous Minecraft GLSL Shader File
3D shader file archive used by Nitrous, a program used for customizing the look of the 3D world in the game Minecraft; saved in a .ZIP format and contains an archive of .FSH, .VSH, and Java .CLASS files; used for enabling the use of GLSL (OpenGL Shading Language) shaders in Minecraft.
To apply Daxnitro's GLSL shader mod to Minecraft, open Nitrous, select, and choose your N2 file. Next, click the button to install the mod.
Nitrous is distributed as a .JAR file. To run it, you can type java -jar nitrous.jar at the Windows Command Prompt. Alternatively, you may be able to just double-click the JAR file to get the program to run.
NOTE: Nitrous is no longer available for download.
Updated: November 24, 2017