Comment 1 for bug 1392864

The OPF is not an extra format, the OPF is used for a backup of the
calibre metadata.db. And has always been present in all versions of
calibre.

Regarding the rest of your bug report, are you saying that if you have a
folder with the files:

file1.epub
file2.mobi
file3.opf
file4.jpg

adding that folder as single book per directory results in no actual
book files being added?