Ctrl+N is for a general new file and therefore it defaults to the General category. If you have a project open and want to add a file to it then use Ctrl+Shift+A which is the keystroke for adding a new file to a project. This will default to the project's category (e.g. C#) and will remember what your last selection was. For example if you recently used a C# source file then it'll default to that. The defaults are per category.