Simple security feature added

[BrianMwangi21]

Since it's a quick upload -> download web app, I thought of a simple security feature.
On upload, a 4-digit random number is generated which can be memorized or written down.
On download, the 4-digit number is requested, if correct, download proceeds.

This helps when :

1. You are not immediately downloading your file,
2. Someone else has the url and you don't want them to download the file and delete it from the system. Hence you can't lose it

Though it works to that point, it would still be nice to redirect the user back to index.php after download is complete. I have tried but to no avail. Any way to help?