Might be better to pipe the input stream directly to the output stream:
This way, you are not caching the entire file in memory before re-transmission. For example, here is how you would write it to a FileStream:
FileUpload fu; // Get the FileUpload object.
using (FileStream fs = File.OpenWrite("file.dat"))
If you wanted to write it directly to another web request, you could do the following:
FileUpload fu; // Get the FileUpload object for the current connection here.
HttpWebRequest hr; // Set up your outgoing connection here.
using (Stream s = hr.GetRequestStream())
That will be more efficient, as you will be directly streaming the input file to the destination host, without first caching in memory or on disk.