Was performing some backup/restore tests with the assignment activity when I've discovered that, at least in the "upload 1 file" assignment type (not sure about the rest), when the teacher is grading the student submission, there is one option to put one "response" file (component/filearea = 'mod_assignment/response).
There are two problems with those files:
1) Logged as student I've not been able to see the response file at all. I see teacher's grade and comments, but not the response file.
2) Those reponse files are being stored with "userid" as itemid in the file pool (instead of "submissionid"). It seems to be exactly the same problem that was reported time ago at
MDL-22609. Perhaps this is the cause for 1) above