Problems in making Notepad Plus Plus the default editor for plain text file types
Normally there are two easy and efficient ways of associating some file types with a program. You can do it in Windows Settings, or right click a file to open it with a program, simultaneously checking the option for this type to always open by this specific program.
However, if you try to use any of these methods to associate, for instance, txt files or ini or css or whatever plain text file type with Notepad Plus Plus, you can be surprised by Windows refusing to accept your choice! No matter how many times you try to make such an association, Windows is stubborn!
Here is how to overcome your problem. Open Notepad Plus Plus “as an administrator” (right click its icon and select this option from the context menu). Then open its Preferences and associate the program with any file types you like.
No need to be exhaustive; add some common types (txt, ini, etc) and add to them whatever you need when you need it. Note that after you associate a file type with N++ from the application’s settings, you may need also to right click the file of that type and use the command to “open it always” with N++! Enjoy!