I'd like to have certain file extensions to be opened with some specific programs. What are the files the edit ? I have found in /usr/share/appplications/default.list but it doesn't work.
Have a look at the file
There is a section
Alternatively, in KDE one can accomplish the same in GUI in File Associations inside System Settings.
To register a MIME type for an extension see freedesktop.org - AddingMIMETutor and stackoverflow - Register file extensions / mime types in Linux.