↑ ↑ ↑
You can drag-drop any 'extra' files into Book Details panel where the cover is shown.
For the books where you've added the files into the book folders, just open the book folder and drag-drop the 'extra' files into Book Details panel, that will record their existence in the database..
FWIW I keep a pro-forma notes file on my desktop, when I want to create some notes for a book I drag-drop it into the Book Details panel. I use a .ONE file which I edit with OneNote.
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