The best way to do this, I guess, is to monitor files and folders outside of QlikView.
A small program or batch (CMD) file checks the availability of this file or folder. Upon detecting the right stuff, it sends an EDX request (there is a command line tool for that, see QMSEDX_commandline in doc http://community.qlik.com/docs/DOC-2650) to your reload task. Configure your reload task to also trigger on receiving an EDX request.
Now schedule your monitoring tool using Windows Task Scheduler to go and check with the preferred frequency and you're on your way.
you can do but with the help of external task.
1. Use the EDX task trigger for the same and write script accordingly. else you can follow the below steps(both) for your requirement but it not exactly what you want.
2. Create a batch file which execute the reload when the required file exist :- Algoritham
if file exist
print Not present
3. Create a external task using above file and sechedule it every 5 min.