Skip setting 'Content-Type' on file upload, when multiple bodies are supported

Describe the bug
Form-style submissions to endpoints that support multiple different content types are missing the necessary "boundary" attribute on the Content-Type header.

POST https://.../items HTTP/1.1
Host: ...
Accept: */*
Accept-Encoding: gzip, deflate
Connection: keep-alive
User-Agent: python-httpx/0.25.2
Authorization: Bearer ...
Content-Type: multipart/form-data
Transfer-Encoding: chunked

Cause
https://github.com/openapi-generators/openapi-python-client/blob/660941dfe23a39ea9a22dd724e1db54afb130d08/openapi_python_client/templates/endpoint_module.py.jinja#L48-L63

In the template above, lines 60-62 have an "if content-type is not multipart/form-data" condition since httpx sets content type and multipart form boundary properly when files are provided. The same condition needs to be applied around line 54, which occurs when multiple body types are supported.

Beginnen Sie mit openapi_python_client/templates/endpoint_module.py.jinja, insbesondere mit der Content-Type-Behandlung um die Zeilen 48-63 und der vorhandenen Multipart-Bedingung. Überprüfen Sie generierte Requests für Endpunkte, die mehrere Body-Typen unterstützen, und betrachten Sie die Arbeit als abgeschlossen, wenn Multipart-Uploads eine Content-Type-Boundary erhalten, ohne die Header-Behandlung von httpx zu überschreiben.