This PR updates the handling of attachments on events.

Resolves: #706

Context:
When encountaring binary data, the client sets a number of placeholders in the first package and then sends the actual data in the packets following it without additional info prepended.

although they state here that
each Socket.IO packet is wrapped in a Engine.IO message packet, so they will be prefixed by the character "4" when sent over the wire.

it does not seem to apply to attachements, as seen in this example:

      socket.send(
        '452-["message",{"_placeholder":true,"num":0},{"_placeholder":true,"num":1}]'
      );
      socket.send(Uint8Array.from([1, 2, 3]));
      socket.send(Uint8Array.from([4, 5, 6]));

      const packets = await waitForPackets(socket, 3);