def download_attachment(type, seq, &callback)
      assert!
      raise "Type mismatch" unless %w(photo video audio).include?(type)
      @connection.communicate(["load_#{type.to_s}", seq], &callback)
    end
def download_attachment(type, seq, &callback)
      assert!
      raise "Type mismatch" unless %w(photo video audio).include?(type)
      @connection.communicate(["load_#{type.to_s}", seq], &callback)
    end
Download an attachment from a message

 @param [type] type The type of an attachment (:photo, :video, :audio)
 @param [String] seq Message sequence number
 @param [Block] callback Callback block that will be called when finished
 @yieldparam [Bool] success The result of the request (true or false)
 @yieldparam [Hash] data The raw data of the request
 @since [0.1.1]