If you want to make some tests. I made a small HTML page with 3 download links :
- Direct download
- Download through a PHP script
- Download through a PHP script hidden by some URL rewriting
In all 3 cases the mimetype is correct : application/epub+zip.
I found out that only the second link is not working. But after each test my Kobo freezed and rebooted.
You can test it yourself : http://cops-demo.slucas.fr/index.html
and tell me if you have the same results.