GuzzleHTTP Async Multipart

I'm using Guzzle HTTP to send requests to my API server. I am also using the async methods ($client->sendAsync($request) compared to $client->send($request)) as I also use the ReactPHP HTTP adapter for non-blocking requests.

However, I'm at the point where I need to send a multi-part request to the server. From what I can see there isn't a way to say that it will be a 'multipart' request. I've tried to manually insert the multipart headers and borders but it didn't seem to work.