File Upload/ Difference Between IIS6 Or IIS7?

Given the following code which is extremely generic, I was hoping someone could tell me a bit about what is going on behind the scenes...

[HttpPost]
public ActionResult Load(Guid regionID, HttpPostedFileBase file)
{
if (file.ContentLength == 0)
RedirectToAction("blablabla.....");

var fileBytes = new byte[file.ContentLength];
file.InputStream.Read(fileBytes, 0, file.ContentLength);
}

Specifically, is the file completely uploaded to the server before my action method is invoked? Or is it the file.InputStream.Read() method call that causes or rather waits for the entire file to upload. Can I do partial reads on the stream and gain access to the "chunks" of the file as it is uploaded? (If the entire fire is uploaded before my method is invoked then it is all a moot point.) Is there any difference between IIS6 or II7 here?