If you are allowed to edit a post (you are the original author or instructor or full access grader or limited access grader) you should be able to remove an attachment as well.

Make sure the attachment remains in the history though. History should only be visible to instructor and full access grader (maybe limited access grader?)

As we discussed in our meeting today, let's allow limited access graders to edit posts, but probably still not allow them to see the author name if the author has requested anonymity. Only full access grader and instructor can see author name when anonymous.

edited on 6/11/19 to add... we also want to allow the addition of new attachments to a post from the edit post interface.