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.

Inizia da openapi_python_client/templates/endpoint_module.py.jinja, in particolare dalla gestione di Content-Type intorno alle righe 48-63 e dalla condizione multipart esistente. Verifica le requests generate per gli endpoint che supportano più tipi di body e considera il lavoro completato quando gli upload multipart ricevono una boundary Content-Type senza sovrascrivere la gestione degli headers di httpx.