Add Custom User Extensions
In Notepad++ navigate to Settings > Style Configurator…
Choose aÂ language on the left side that you want your own extension to match (i.e. if I want .config files to style like XML then I’d choose XML). In the User ext. field near the bottom, enter in your custom user extension without the dot in front of it (e.g. config for *.config files). Now restart Notepad++ and it will map that new extension to the style you’ve chosen.