As @davidfor just said, nothing ever feeds you a .kepub, certainly not Kobo itself.
Calibre only uses it *internally* as a file extension if you happen to store KePubs in your library, a decision which I suspect was made purely for simplicity's sake.
As soon as you Send To Device, it's sent as a .kepub.epub. Which I'm pretty sure is also another Calibre convention: I haven't checked, but I suspect that even without a kepub mention in the filename, your device would figure out it's a KePub and not a straight ePub
.
Code:
Calibre Library:
'/home/niluje/Calibre/Douglas Adams/The Ultimate Hitchhiker'\''s Guide to the Galaxy (79)/The Ultimate Hitchhiker'\''s Guide to the Gal - Douglas Adams.kepub'
->
Device:
/mnt/onboard/Adams, Douglas/Ultimate Hitchhiker_s Guide to the Galaxy, The - Douglas Adams.kepub.epub
(Because, again, Kobo's own KePubs don't even *have* a file extension).
Code:
A bunch of files whose name if just an UUID with no extension in /mnt/onboard/.kobo/kepub