Does the multipart HTTP push enable splitting up a large software image and then sending the same to a Redfish service via multiple POSTs, effectively enabling streaming a large software image in parts? Or, do the multiple parts here really mean different portions of the form (update parameters, binary file, etc.)?
HTTP multipart only means different forms in the request body, where each form can have its own MIME type. RFC7578 describes this further, but in short it's still a single HTTP request; it does not break up a binary into multiple requests.