multipart files

I believe you are talking about users sending files via web API to Appian. As far as I have seen there is no straightforward way to do this. the document upload web API pattern expect the whole body to be in a binary file type. 

Is your file sent as a binary of base64? if it is base64 we may be able to do it via storing it in db and generating a document like that  (), else what I recommend is you can create an intermediary integration object that accepts base64 value which can be called inside the web API and convert the base64 string to a document via the capability of the integration object in Appian.

if the document is sent as a binary file then I'm not sure it will work in Appian at the moment when sending as a multi-part request.

Probably someone else will be able to help you if the request contains a binary file.