I use PAPERBOOK files (plain text with a renamed extension) for my physical book collection. Currently I keep a dummy file in the documents folder and use "add files to selected book records..." to put it in.
A few weeks ago I noticed the "add an empty file to selected book records" option in the menu. I tried looking up how to add other formats to it but I could not find any information.
Is it possible for a user to add another format to that dialog? I suspect it might be hardcoded in.