Handle upload/download raw files

[chriskapp]

Currently our SDK can only work with JSON payloads. We should also add support for file up/download. This means for uploads our client SDK needs to send an multipart/form-data request to the endpoint. For downloads we simply need to return the raw body content without trying to convert the body into a specific object. For file up/download we are not able to describe the body payload so we need some kind of dummy schema which tells the user that this is a raw file.