Windows file locking is not mandatory, it's up to the application to use or not use.
Example: if I change the Author of a book whilst Notepad++ has the book's TXT format file open, it pops an error message right after calibre does the move, viz:
Sigil & Calibre Editor are similar: they pop an error message when the EPUB that is being edited is saved with a Ctrl+S.
I know I suggested an "Attachments" folder, but on thinking about it I wondered if it could open a can of worms if users were allowed to put .lnk files, windows reparse points (symlinks, folder junctions, mount points etc), hardlinks etc, into the folder.
BR