Another issue with your code...
if (System.IO.File.Exists(Server.MapPath(folder_from_config) + "\\" + System.IO.Path.GetFileName(FileUpload1.FileName)))
System.Windows.Forms.MessageBox.Show("File already exists. Do you want to overwrite");
You shouldn't be using System.Windows.Forms.MessageBox in an ASP.Net application. Once that is deployed onto a web server, the client who uploads the file wont see the message box
You are performing a POST operation when sending a file - If you want the client to see some some of message, you have to send a Response. You can't just use a MessageBox!