class Restforce::Middleware::Multipart

def create_multipart(env, params)
  boundary = env[:request][:boundary]
  parts = []
  # Fields
  parts << Faraday::Parts::Part.new(
    boundary,
    'entity_content',
    params.reject { |k, v| v.respond_to? :content_type }.to_json,
    JSON_CONTENT_TYPE
  )
  # Files
  params.each do |k, v|
    parts << Faraday::Parts::Part.new(boundary,
                                      k.to_s,
                                      v) if v.respond_to? :content_type
  end
  parts << Faraday::Parts::EpiloguePart.new(boundary)
  body = Faraday::CompositeReadIO.new(parts)
  env[:request_headers]['Content-Length'] = body.length.to_s
  body
end