you need to do like below code
string filename = Path.GetFileName(FileUpload1.PostedFile.FileName);
string contentType = FileUpload1.PostedFile.ContentType;
Stream fs = FileUpload1.PostedFile.InputStream;
BinaryReader br = new BinaryReader(fs);
byte[] bytes = br.ReadBytes((Int32)fs.Length);
Response.Clear();
Response.Buffer = true;
Response.Charset = "";
Response.Cache.SetCacheability(HttpCacheability.NoCache);
Response.ContentType = contentType;
Response.AppendHeader("Content-Disposition", "attachment; filename=" + fileName);
Response.BinaryWrite(bytes);
Response.Flush();
Response.End();
code is not tested you need to check