File Uploading in asp.net

Question

Aslam-0-Alaikum

   if (FileUpload1.PostedFile != null)

        {

            if (FileUpload1.PostedFile.ContentType == "jpg" || FileUpload1.PostedFile.ContentType == "gif" || FileUpload1.PostedFile.ContentType == "png")

            {

                if (FileUpload1.PostedFile.ContentLength == 20)

                {

                    string fileName = FileUpload1.FileName;

                    FileUpload1.SaveAs(@"D:\" + fileName);

                    Response.Write("File Uploaded");

                }

                else

                {

                    Response.Write("Image must 20 kb");

                }

            }

            else

            {

                Response.Write("File must be Image");

            }

        }

        else Response.Write("Please select file to upload");

it is not working idon't know but why ..............?

And

   FileUpload1.PostedFile.SaveAs(Server.MapPath("~/App_Data") + System.IO.Path.DirectorySeparatorChar + FileUpload1.PostedFile.FileName);

    ystem.IO.Path.DirectorySeparatorChar   What is mean this ....?