Set the Form property allowDrop = True;
In the Form event C # tutorial

private void Form1_DragDrop(object sender, DragEventArgs e)
        {
            string localFilePath = ((System.Array)e.Data.GetData(DataFormats.FileDrop)).GetValue(0).ToString();//Full path of the file
            GetExtension(localFilePath);//file suffix name
            if (extension == ".cbd")
            {
 　　　　　　　　//todo you code
            }
        }

        private void Form1_DragEnter(object sender, DragEventArgs e)
        {
           
            string localFilePath = ((System.Array)e.Data.GetData(DataFormats.FileDrop)).GetValue(0).ToString();
            string extension = System.IO.Path.GetExtension(localFilePath);
            if (string.Compare(extension, ".cds", true) == 0)
            {
                e.Effect = DragDropEffects.Move;
            }
            else
            {
                e.Effect = DragDropEffects.None;
            }
        }