Okay, I did a little more digging. I am using 2.4.8, and this issue, https://tracker.moodle.org/browse/MDL-38452, tells me that the trouble is probably with my attempts to use the "Login as..." functionality. Specifically see Frederic Massart's comment of March 16th to explain why this is. Apparently my problem would be fixed in 2.5
For now, I'll have to check to see with the student if they can upload a file.