bootmgr.efi.mui

Multilingual User Interface resource file Reference

A .mui file contains language resources for a Windows program. The file has the PE format, but it holds only resource data, not code. Windows loads the correct .mui file for the language of the user. Each supported language has its own .mui file for one binary. The mechanism is part of the Windows Multilingual User Interface system.

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.