When you can and can’t preview
I was in the Teach Tab today, looking through a course. You cannot preview HTML files (that are stored in a folder) from the Teach Tab. You can’t tell at all that they are HTML files, and I assumed they were PDFs. My bad. My other error was that you can click on a file too look at in while in the Teach Tab.
Lesson learned: and one I tell people all the time. Check what tab you are in.