Make TaskCompletionSource<T>.Task run in background from asp.net mvc request

Question

In one action of my MVC 4 apps, I have a call:

public ActionResult Test()
{
   DownloadAsync("uri","file path");
   return Content("OK");
}

DownloadAsync return a Task and I expect to see the DownloadAsync run in background. But I always see that MVC only response when the Task of DownloadAsync is completed (means that need wait for download complete before response). If I wrap the async call in to Task.Run() or Task.Factory.StartNew(), then it works as my expectation. Here's method DownloadAsync:

private Task DownloadAsync(string url, string originalFile)
{
    var tsc = new TaskCompletionSource<bool>();

    var client = new WebClient();
    AsyncCompletedEventHandler completedHandler = null;
    completedHandler = (s, e) =>
    {
        var wc = (WebClient)s;
        wc.DownloadFileCompleted -= completedHandler;

        if (e.Cancelled)
        {
            tsc.TrySetCanceled();
        }
        else if (e.Error != null)
        {
            tsc.TrySetException(e.Error);
        }
        else
        {
            tsc.SetResult(true);
        }
        wc.Dispose();
    };
    client.DownloadFileCompleted += completedHandler;
    client.DownloadFileAsync(new Uri(url), originalFile);

    return tsc.Task;
}

So my question are:

1. Why MVC request need wait for complete Task in this case? Is there any special for Task created by TaskCompletionSource<T>?

2. How to make the Task of DownloadAsync run in background without pause the response of MVC request?

Thanks,