The accepted files type pop up does not expand and does not display the options available when the following steps are used.

1. Create a course.
2. Enrol students.
3. Go to the course and turn edit mode to on.
4. Add an Assignment activity.
5. Go to submission types.
6. Click Choose for the "Accepted file types"
7. Click any of the "Expand" links in the pop up, but make no changes.
8. Click "save".
9. Click again the Choose for the "Accepted file types".
10. Click any of the "Expand" again.

RESULT: The UI will not expand and will not show any options.

Work-around is to redo the choosing and the steps again, but it is inconsistent.

Replicated as well at https://school.moodledemo.net/