not a valid virtual path - when trying to return a file from a url - asp.net-mvc-3

Question

We download a file from our CdN and then return a url to that downloaded file to the user. I'm trying to get this implemented so that when a user clicks the download buttton, it goes and test that url to that downloaded file then forces a save prompt based on that local url.
So for example if there is a button called download on the page for a specific .pdf, we ultimately have code in our controller going to the cdn and downloading the file, zipping it then returning a url such as: http://www.ourLocalAssetServer.com/assets/20120331002728.zip
I'm not sure if you you can use the File() method to return the resource to the user as to cause a save prompt when you have a url to the file, not a system directory virtual path.
So how can I get this working with the url? I need the download button to ultimately force a save prompt on their end given a url such as what is generated per this example above? Not I am using POST, not a GET, so not sure which I should use in this case either besides this not working overall to force a save prompt. It is hitting my GetFileDownloadUrl but ultimately errors saying it's not a virtual path.
Here's my code:
#foreach (CarFileContent fileContent in ModelCarFiles)
{
using (Html.BeginForm("GetFileDownloadUrl", "Car", FormMethod.Get, new { carId = Model.CarId, userId = Model.UserId, #fileCdnUrl = fileContent.CdnUrl }))
{
#Html.Hidden("userId", Model.UserId);
#Html.Hidden("carId", Model.CarId);
#Html.Hidden("fileCdnUrl", fileContent.CdnUrl);
<p><input type="submit" name="SubmitCommand" value="download" /> #fileContent.Name</p>
}
}
public ActionResult GetFileDownloadUrl(string fileCdnUrl, int carId, int userId)
{
string downloadUrl = string.Empty;
// take the passed Cdn Url and go and download that file to one of our other servers so the user can download that .zip file
downloadUrl = GetFileZipDownloadUrl(carId, userId, fileCdnUrl;
// now we have that url to the downloaded zip file e.g. http://www.ourLocalAssetServer.com/assets/20120331002728.zip
int i = downloadUrl.LastIndexOf("/");
string fileName = downloadUrl.Substring(i);
return File(downloadUrl, "application/zip", fileName);
}
error: not a valid virtual path

Answer 1

This won't work except the zip file is in your virtual path.
The File method you have used here File(string, string, string) expects a fileName which will be used to create a FilePathResult.
Another option would be to download it (using WebClient.DownloadData or DownloadFile methods) and passing either the byte array or the file path (depending on which you choose).
var webClient = new Webclient();
byte[] fileData = webClient.DownloadData(downloadUrl);
return File(fileData, "application/zip", fileName);
And the lines where you get the index of "/" just to get the filename is unnecessary as you could have used:
string fileName = System.IO.Path.GetFileName(downloadUrl);