Here is the concept:
A user has options: Red, Green, and Blue
If the user pick "Red", a file will be created, "Red" is written to it, and then the file is offered to be download.
Here is what I come up with by myself. If you guys know better method, let me know. Also let me know if my system could potentially be abused.
I will allow, say, 10 downloads at a time. So I'm gonna have file01-file10. When user enters his selection, I'm gonna check if there is a file among the ten that is not opened. If there is, selection is written to the file and offered to be downloaded. If not, prompt to try again later.
EDIT:
The filename has to be the same. So my quick and dirty solution is to create each of the ten files in ten different folders. Any thoughts?