Try this. Hope this what you want.
if (FileUpload1.HasFile)
{
string directoryPath = "~/Mail_Attachments/" + txtSubject.Text;
string filePath = directoryPath + "/" + FileUpload1.FileName;
if (!Directory.Exists(MapPath(directoryPath)))
System.IO.Directory.CreateDirectory(MapPath(directoryPath ));
FileUpload1.SaveAs(MapPathfilePath ));
temp = filePath ;
}