The only part of the webmachine_multipart API currently in use and tested is get_all_parts/2. The streaming form API (stream_parts/2) is much more experimental.
The latter function needs to work as documented and have demonstrative tests in webmachine_multipart.erl showing an example of usage. Ideally it should take advantage of the streamed body mechanism, only reading sufficient body chunks at any time to produce the requested form parts.
(<<user benoitc>> attempted to use this function and ran into difficulty)