workbook.SaveAs(stream);
stream.Seek(0, SeekOrigin.Begin);

return this.File(
                    fileContents: stream.ToArray(),
                    contentType: "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet",
                    fileDownloadName: "FileName.xlsx"
                );

// Get the response.
WebResponse response = request.GetResponse();
// Display the status.
Console.WriteLine(((HttpWebResponse)response).StatusDescription);
// Get the stream containing content returned by the server.
dataStream = response.GetResponseStream();
// Open the stream using a StreamReader for easy access.
StreamReader reader = new StreamReader(dataStream);
// Read the content.
string responseFromServer = reader.ReadToEnd();

var memoryStream = new MemoryStream();

dataStream.CopyTo(memoryStream);

return this.File(
                    fileContents: memoryStream.ToArray(),
                    contentType: "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet",
                    fileDownloadName: "FileName.xlsx"
                );

StreamReader reader = new StreamReader(dataStream); 

dataStream.CopyTo(memoryStream);