I am not sure on share point side, but you could encode you’re file document on Mendix on base64 and send it then by Rest Service in the message. Maybe you take a look in the Module Community Commons for some helpful Java actions.
I have implemented this a couple of weeks ago for a client.
I do not know if you need some form of authentication and the steps that I wrote down below are probably very specific for my setup but it may help you get in the right direction. Here are the steps that I have implemented:
Maybe I forgot something to mention so feel free to ask questions.
Some extra documentation: