[Multi-part upload] TypeError: Object of type BytesIO is not JSON serializable

I have a generated client code for FastAPI multi-file upload endpoint which I'm using like this:

file_names = ['fd1.py', 'fd2.py']
files = []
for f in file_names:
    with open(f, 'rb') as fh:
        buf = BytesIO(fh.read())
        files.append(buf)

multipart_data = BodyUploadFeatureDefinitionFeatureDefinitionPost(
    files=[File(payload=files[i], file_name=file_names[i], mime_type='text/x-python') for i in range(len(files))]
    tags=[] # not relevant
)

res = upload_feature_definition_feature_definition_post.sync(
    client=self.client,
    multipart_data=multipart_data
)

I get an error:

Traceback (most recent call last):
  File "/Users/anov/IdeaProjects/svoe/client/featurizer_client/featurizer_client.py", line 77, in <module>
    client.register_feature_definition(
  File "/Users/anov/IdeaProjects/svoe/client/featurizer_client/featurizer_client.py", line 63, in register_feature_definition
    res = upload_feature_definition_feature_definition_post.sync(
  File "/Users/anov/IdeaProjects/svoe/client/fast_api_client/api/default/upload_feature_definition_feature_definition_post.py", line 146, in sync
    return sync_detailed(
  File "/Users/anov/IdeaProjects/svoe/client/fast_api_client/api/default/upload_feature_definition_feature_definition_post.py", line 103, in sync_detailed
    kwargs = _get_kwargs(
  File "/Users/anov/IdeaProjects/svoe/client/fast_api_client/api/default/upload_feature_definition_feature_definition_post.py", line 40, in _get_kwargs
    multipart_multipart_data = multipart_data.to_multipart()
  File "/Users/anov/IdeaProjects/svoe/client/fast_api_client/models/body_upload_feature_definition_feature_definition_post.py", line 67, in to_multipart
    files = (None, json.dumps(_temp_files).encode(), "application/json")
  File "/Users/anov/miniconda3/envs/env_py_3-10/lib/python3.10/json/__init__.py", line 231, in dumps
    return _default_encoder.encode(obj)
  File "/Users/anov/miniconda3/envs/env_py_3-10/lib/python3.10/json/encoder.py", line 199, in encode
    chunks = self.iterencode(o, _one_shot=True)
  File "/Users/anov/miniconda3/envs/env_py_3-10/lib/python3.10/json/encoder.py", line 257, in iterencode
    return _iterencode(o, 0)
  File "/Users/anov/miniconda3/envs/env_py_3-10/lib/python3.10/site-packages/frozendict/__init__.py", line 32, in default
    return BaseJsonEncoder.default(self, obj)
  File "/Users/anov/miniconda3/envs/env_py_3-10/lib/python3.10/json/encoder.py", line 179, in default
    raise TypeError(f'Object of type {o.__class__.__name__} '
TypeError: Object of type BytesIO is not JSON serializable

Generated File class:

@attr.s(auto_attribs=True)
class File:
    """Contains information for file uploads"""

    payload: BinaryIO
    file_name: Optional[str] = None
    mime_type: Optional[str] = None

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

Looks like generated code tries to call json.dumps() on tuple representation of File, which has BinaryIO field, this causes the exception.

What am I doing wrong?

Start with the generated body_upload_feature_definition_feature_definition_post.py model and its to_multipart() method, then trace the _get_kwargs() call in upload_feature_definition_feature_definition_post.py. Reproduce the multipart request using the shown File and BytesIO values; done means valid file payloads no longer fail during JSON serialization.