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.

openapi_python_client/templates/endpoint_module.py.jinja から始め、特に48-63行付近の Content-Type の処理と既存の multipart 条件を確認してください。複数の body タイプをサポートするエンドポイントについて生成された requests を検証し、httpx のヘッダー処理を上書きせずに multipart アップロードが Content-Type boundary を受け取ることを確認できたら、作業完了とみなしてください。