But even in a simple assignment you can
1. upload the file thru the files admin block, copy the link, and paste the link into the comment box so the student can download the amended file.
2. Or, simply click the create a link button above the comment button, click browse, click upload a file, and go from there.
The smart thing to do, though, is to correct all the files, zip them, upload through admin > files > unzip to a specific folder (neatness counts!), and then, go to the comment box and make a link to each student's file. Fewer clicks!