That anecdote is inline with my explanation: the student manually changed the file extension on disk, then uploaded that. That would pass all of the checks, but

That anecdote is inline with my explanation: the student manually changed the file extension on disk, then uploaded that. That would pass all of the checks, but it would still be in the wrong format, as manually changing the file extension does not cause a conversion to take place on disk, and that probably prevents the automatic conversion of HEIC to jpeg the iPhone itself does on such input fields.