private void treeView1_DragEnter(object sender, DragEventArgs e)
        {
            e.Effect = DragDropEffects.Copy;
        }

        private void treeView1_DragDrop(object sender, DragEventArgs e)
        {
            TreeNode t = treeView1.GetNodeAt(treeView1.PointToClient(new Point(e.X, e.Y)));
            if (t == null)
                return;
            string[] FileList = (string[])e.Data.GetData(DataFormats.FileDrop, false);
            foreach (string File in FileList)
                t.Nodes.Add(File);
        }