As a workaround/heuristic, maybe you could reconstruct it yourself from the lines such as --MIMEBoundaryurn_uuid_5922B37D7F43649CB512578951566415053 (i.e. strip off the "--"), and simply prepend the required multipart header to the response before passing it to the parser...
Better yet, of course, fix the response in the first place (if that's an option).
Ok so then this is definitely encoded wrong by the provider. I finally found some code on this (very very large internet company)'s confusing developer area. The code they have basically uses regex's to chop it all up. Its nice to know they are following standards...
Thanks for the help!