To add more file creation options, you use a Templates folder.
Just create a folder named "Templates" in your home directory. Then use rnote to save an empty rnote file in that directory.
Your right click menu to create new files should now get an option to "create" rnote files (which are really just copies of the file in the Templates folder).
You can add whatever types of files you want in Templates, and they don't have to be empty.
