Upload file logic seems to be wrong on 3.1 schema

Describe the bug
I just upgraded my API, and the schema changed from 3.0 to 3.1. The uploads suddenly broke. It seems that the code generated for the body class now looks like this:

    file: str
 
    ...
    def to_multipart(self) -> types.RequestFiles:
        files: types.RequestFiles = []

        files.append(("file", (None, str(self.file).encode(), "text/plain")))

        for prop_name, prop in self.additional_properties.items():
            files.append((prop_name, (None, str(prop).encode(), "text/plain")))

        return files

While the old looked like this:

    file: File

    python
    def to_multipart(self) -> types.RequestFiles:
        files: types.RequestFiles = []

        files.append(("file", self.file.to_tuple()))

        for prop_name, prop in self.additional_properties.items():
            files.append((prop_name, (None, str(prop).encode(), "text/plain")))

        return files

Where the File object is this:

@define
class File:
    """Contains information for file uploads"""

    payload: BinaryIO
    file_name: str | None = None
    mime_type: str | None = None

    def to_tuple(self) -> FileTypes:
        """Return a tuple representation that httpx will accept for multipart/form-data"""
        return self.file_name, self.payload, self.mime_type

OpenAPI Spec File
A link to an OpenAPI document which produces this issue. Ideally, write a minimal reproduction only containing the problematic pieces.

The API spec for the body (new 3.1):

          "file": {
            "type": "string",
            "contentMediaType": "application/octet-stream",
            "title": "File"
          }

Old (3.0) schema:

          "file": {
            "type": "string",
            "format": "binary",
            "title": "File"
          }

Desktop:

• OS: [e.g. macOS 10.15.1]: MacOS (M4) 26.3.1
• Python Version: [e.g. 3.8.0]: 3.11.14
• openapi-python-client version [e.g. 0.1.0]: 0.28.3

Additional context
I have some code pre-massaging around a different name-related issue and adding this to the pre-processing before this tool is called fixed it, but it's definitely more of a temporary workaround type of thing:

for prop in props.values():
      if prop.get("contentMediaType") == "application/octet-stream":
          prop["format"] = "binary"

首先重現根據所提供的 OpenAPI 3.1 schema 產生的 body 類別，並使用 format: binary 將其與 3.0 schema 進行比較。追蹤 contentMediaType: application/octet-stream 在產生過程中的解讀方式；完成的條件是產生的欄位使用 File，且 multipart 序列化如同舊輸出一樣呼叫 to_tuple()。