See this thread:
https://www.mobileread.com/forums/sh...d.php?t=358636
How to Use and Manage Data Files
Instead of manually digging through your computer's file system, Calibre provides a dedicated tool to manage this folder directly.
Select the book in your Calibre library.
Click the Edit metadata button's drop-down arrow and choose Manage data files.
In the pop-up window, you can view, add, or delete associated files (e.g., PNG, Word docs).