[BUG] [Python] multipart with mixed content

Created by: rienafairefr

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?

Description

The format for multipart/form-data content can specify different types of content in it, this can be useful to for example attach a JSON metadata object to a bunch of uploaded files, like:

requestBody:
  content:
    multipart/form-data:
      schema:
        type: object
        properties:
          metadata:
            description: metadata
            $ref: '#/components/schemas/Metadata'
          files:
            type: array
            items:
              type: string
              format: binary

But the generated client in Python cannot handle this kind of content format. The metadata is not serialized before being included in the multipart form data, just converted to a dict, and urllib3/urlopen afterwards can't do anything to include that in the multipart data.

openapi-generator version

master 550774a6

Suggest a fix

The parameter should be json.dumps somewhere in the chain, probably in api_client before being sent to rest Bonus would be to send the multipart with each part having a different Content-Type header (cf RFC1867), but it's not mandatory