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.

从 openapi_python_client/templates/endpoint_module.py.jinja 开始，尤其检查第 48-63 行附近的 Content-Type 处理以及现有的 multipart 条件。验证支持多种 body 类型的 endpoint 所生成的 requests，并在 multipart 上传获得 Content-Type boundary 且不覆盖 httpx 的 header 处理时，视为工作完成。