def upload(local_file, remote_file)
      raise ConnectionClosed.new('Connection is closed.') unless @ssh
      sftp.upload!(local_file, remote_file)
    end
def upload(local_file, remote_file)
      raise ConnectionClosed.new('Connection is closed.') unless @ssh
      sftp.upload!(local_file, remote_file)
    end
Uses SFTP to upload a single file to the host.