Android : Simulate form post using http client in Android app?

So, I'm currently developing an app for a service which has a json-based (unfortunately) read only API. Retrieving content is no problem at all, however the only way to post content is using a form on their site which location is a PHP script. The service is open source so I know which fields the form expects, but whatever I send, it always results in a BAD REQUEST.

I captured the network traffic inside my browser and as far as I can see, the browser constructs a multipart form request, however when I copy the request and invoke it again using a REST client, a BAD REQUEST gets returned.

Is there a way to construct a http request in Android that simulates a form post?