Shame MS, couldn’t adjust it through Settings / Apps / Default Apps as you don’t allow one letter extensions there. -sigh-
So, Windows 11 does not allow you to adjust a one letter file extension.
This is annoying, I had to play around with FileTypesMan – Alternative to ‘File Types’ manager of Windows (nirsoft.net) to re-register .c and .h files that I like to view or edit with Notepad++ on windows.
When I messed about with the Nirsoft tool I could then double-click the .c and .h files an re-register them with one of my favorites apps.