There is only one file type that uses the .NI extension:
Inform 7 Source Code File
An NI file contains Inform 7 (I7) source code, and possibly Inform 6 (I6) source code, that can be compiled into an Interactive Fiction (IF) program by Inform 7, an IDE used to create IF programs. It stores plain text with Unicode UTF-8 encoding in I7 or I6 syntax that outlines the rules and behaviors for how a program can be played along with descriptions of playable game objects.
You can open an NI file with Inform 7, then compile it by selecting "Go!" from the user interface.
Since NI files are saved in plain text, you can also open them with a plain text editor, such as Microsoft Notepad (Windows) or Apple TextEdit (macOS), and edit them. Also, Microsoft Visual Studio Code (multiplatform) is a useful source code editor that includes helpful syntax editing capabilities for viewing and editing NI files.
Updated: August 27, 2020