The Multipart middleware converts a Faraday::Request#body Hash of key/value
pairs into a multipart form request, but only under these conditions:
- The request's Content-Type is "multipart/form-data"
- Content-Type is unspecified, AND one of the values in the Body responds to
  #content_type.

WWW: https://github.com/lostisland/faraday-multipart