Stop From Uploading Posted File?

I'm developing a httpHandler, in my source code if some conditions aren't met i would like to stop uploading file to the server ( for example file extension is not allowed)

When I try to quit ProcessRequest function with return null;, asp.net tries and uploads the file automatically before I quit the httpHandler...

How can I stop asp.net to automatically upload posted files?

Here is the list of thing I've tested so far and no luck:

public void ProcessRequest(HttpContext context) {
var worker = (HttpWorkerRequest)provider.GetService(typeof(HttpWorkerRequest));
..........
worker.CloseConnection();
context.Response.End();
Environment.Exit(0);
return null;
}