How to add an option to create a new file with a specific extension in right click menu
You may want to use Templates directory (or how is called in your language) from your home directory in this sense. Files from this folder will appear in the New Document menu. So, add the files with the extension what you want in this folder and you can create them after files with the same extension from the right click menu:
Moreover, if you add some predefined text in these files from Templates folder, the new files which you will create them from the right click menu will contain that text.
See also: What is the "Templates" folder in the home directory for?