notepad.exe.mun

Language-neutral resource file

A .mun file contains the language-neutral resources of a Windows system binary. Since Windows 10 version 1903, the system keeps icons, images, and other neutral resources in .mun files in the SystemResources directory. The file names end in .dll.mun or .exe.mun, after the binary that owns the resources. The language-specific resources stay in .mui files. The resource loader combines both when a program loads.

A file extension is only a part of the file name. It does not give a guarantee about the format or the content of a file. But on Windows, the extension selects the program that opens the file.