Swagger: multipart with json and file part

[jojoob]

I've a resource excepting multipart form data with one part named "_json" with application/json encoding. The content is a json object described by the "post_schema". The other part is an optional binary file.

Want I want to achieve is the "_json" paramater should be rendered in swagger UI using the provided schema like it is done when using auto generated config via "@api.expected()".

I tried a lot but don't got it to work. I come somewhat close with this code:

params={
    '_json': {
        # 'required': True,
        'in': 'formData',
        'type': 'object',
        # 'format': 'application/json',
        'schema': post_schema.get_dereferenced_schema(),
        'description': 'The task log message.'},
    'attachment': {
        'in': 'formData',
        'type': 'file',
        'format': 'binary',
        'description': 'An optional file attachment to the task log.'}}

(When I set required to True swagger UI will reject the execution with "Required field is not provided")

It looks like this:

But I want that the "_json" parameter get rendered like this:

[jojoob]

When using 'type': 'string' I get around the 'required': True problem. But then the input is rendered as a text input.