Reply To: Sorry, this file extension is not permitted for security reasons.
Just posting the solution to our original error problem.
We run a WP network (multi-site) and I discovered in the main newtwork settings (…/wp-admin/network/settings.php) that there is a section on “Upload settings”. In that section there is a part where you specify file types and mp4 wasn’t on the list.
Once I did, the error goes away.
So we’re just back to the 256mb upload restriction which has suppered our plans!