[Python] Issue with Multipart/Form-Data Requests

Description

I am encountering an issue with Swagger UI when using the Python generated code to perform multipart/form-data requests.
The server receives que request, but it fails validating the data. This happens both using the client code or the curl command.

http://localhost:8080/test validation error: '{\r\n  "name": "string",\r\n  "age": 0\r\n}' is not of type 'object' - 'body'
127.0.0.1 - - [12/Dec/2023 12:46:37] "POST /test HTTP/1.1" 400 -

Looking at the examples in the documentation, the root object is called body, and the file is binary so it is encoded octet-stream by default, so I don't see any errors here, but for some reason the server cannot validate it correctly .

Swagger-codegen version

https://editor-next.swagger.io/
OS: Ubuntu 22.04.2 LTS
Python: Python 3.10.12
Python packages:
swagger-ui-bundle 0.0.9
connexion 2.14.2
urllib3 2.1.0
Flask 2.2.5
six 1.16.0

Swagger declaration file content

openapi: 3.0.1
info:
  title: Test
  version: 1.0.0
servers:
  - url: http://localhost:8080
paths:
  /test:
    post:
      requestBody:
        content:
          multipart/form-data:
            schema:
              type: object
              properties:
                body:
                  type: object
                  properties:
                    name:
                      type: string
                    age:
                      type: integer
                file:
                  type: string
                  format: binary
      responses:
        '200':
          description: OK

Steps to reproduce

It can be reproduced from python code or curl command:

from swagger_client.configuration import Configuration
from swagger_client.rest import ApiException
from swagger_client.api_client import ApiClient
from swagger_client.api.default_api import DefaultApi
from swagger_client.models import TestBody

configuration = Configuration()
configuration.host = "http://localhost:8080"

api_client = ApiClient(configuration)
api_instance = DefaultApi(api_client)

body = TestBody(
    name="test", 
    age=1
)
file="sample.bin"
try:
    api_response = api_instance.test_post(
        body=body.to_str(), # using .to_dict() method causes a TypeError exception on client side.
        file=file
    )
    print(api_response)
except ApiException as e:
    print("Exception when calling DefaultApi->test_post: %s\n" % e)

curl -X 'POST' \
  'http://localhost:8080/test' \
  -H 'accept: */*' \
  -H 'Content-Type: multipart/form-data' \
  -F 'body={
  "name": "string",
  "age": 0
}' \
  -F 'file=@sample.bin;type=application/octet-stream'

Start with the supplied OpenAPI declaration and the generated Python entry points in api/default_api.py and api_client.py, then reproduce the multipart request using the shown Python and curl examples. Compare how the body and file parts are sent and verify the request against the server's schema validation. Done means the generated client and curl request are accepted without the reported body validation error.