Mui extension stands for MultiLanguage Resource - source file. You can open mui files with Windows Service Pack 1 Muisetup.

These are the files that include sources for Windows 2000 and Windows XP. It gives a chance to use Windows system in different languages. You may add extra languages by using Muisetup.exe application. 

Many Mui files are found in :
" Windows/Mui/FallbackLangID/*.mui " folder.
Some of them may be associated with significant softwares. Such as: Notepad.exe.mui

What to use mui

Windows Service Pack 1 Muisetup

which page related to mui file

mui Related Searches

You can check related links about mui fileextension.