One thing that everyone keeps forgetting is that Kobo doesn't actually support sideloading kepubs for everyday reading. And, their information page for adding books via Dropbox,
https://help.kobo.com/hc/en-us/artic...r-with-Dropbox, has a link to the supported formats,
https://help.kobo.com/hc/en-us/articles/360017763713.
But, I'm pretty sure someone else has already tested that with the extension as .kepub.epub, the books will downloads as kepubs.