Multipart requests

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Hello!

I did not found standard way to describe a multipart request. Is it supported?

Currently i had to write something like this to let uploaded files work:

```
request_body_description = {
"content": {
"multipart/form-data": {
"schema": {
"type": "object",
"properties": {"logo": {"type": "string", "format": "binary"}},
}
}
}
}

@bp.route("/update-logo")
class UpdateLogo(MethodView):
@bp.response(code=204)
@bp.doc(requestBody=request_body_description)
def post(self):
file = flask.request.files["logo"]
filename = secure_filename(file.filename)
binary = file.read()

... do something with file ...
```

This code allows swagger to render input type="file" with name "logo" and send multipart request when executing request from web interface.

Am i missing something?

Thanks!

Start by tracing how @bp.doc handles requestBody descriptions and how the shown flask.request.files usage is represented in the generated OpenAPI document. Compare existing request-body handling and determine what standard multipart support should look like. Done means callers no longer need the manual description and Swagger renders and sends the file input correctly.

Written by the indexing model from the issue text.