Thanks for the reply, but that doesn't solve my problem. I've read about that. The feedback files are named with the usernames of the students and each student gets the correct feedback file. However, that is done when assignments submitted are being graded.
What I want is different. For example, I am uploading 127 files which contain instructions and problems that students solve. The contents vary from one file to another that's why they have to be in separate files. Each student downloads his problem file, solves the problems and submit. It would be good if he cannot see the problem files of other students.