Send POST request with JSON data and file C#

I have a backend on Flask.
I created API call for uploading file and JSON data.
I created a function for testing this API call that doesn't work too.
I can't find how to send request with file and JSON data. Everything that I tried is failed and didn't enter to the page on backend.
I need your help to find the examples to make the request in C# .NET and python.
Test function:

def test_upload_deidentified_file(self, client, headers):
    url = '/upload_deidentified_file'
    file_path = f"{os.path.dirname(os.path.abspath(__file__))}/../../../../files/test_file.mkv"
    with open(file_path, 'rb') as file:
        data = {
            "filename": "test_file.mkv",
            "campaign_name": "Test Campaign Name",
            "session_id": "12345678-1234-1234-1234-123456789123",
            "activity_id": "12345678-1234-1234-1234-123456789123",
            "tags": ["video"],
            "machine_name": "Test Machine Name",
            "creation_date": "2021-01-01",
        }
        file_data = FileStorage(stream=file, filename='test_file.mkv')
        response = client.post(url,
                               data={'json_data': json.dumps(data),
                                     'file': file_data},
                               headers=headers)
    json_data = response.get_json(silent=True)
    logger.info(f"json_data: {json_data}")
    assert response.status_code == HTTPStatus.OK
    assert isinstance(json_data, dict)

Backend:

app = Flask(__name__)
storage_folder = r"path_to_the_storage_folder"


@app.before_request
def before_request():
    message = {
        "remote_address": request.remote_addr,
        "path": request.path,
        "request_method": request.method,
        "headers": request.headers,
        "data": request.data if request.data else None,
        "args": request.args if request.args else None,
        "form": request.form if request.form else None,
        "json": request.json if request.json else None,
    }
    logger.debug(f"Request: {message}")


@app.after_request
def after_request(response):
    message = {
        "remote_address": request.remote_addr,
        "path": request.path,
        "request_method": request.method,
        "status": response.status,
        "response_data": response.data
    }
    logger.debug(f"Response: {message}")
    return response


@app.route('/upload_deidentified_file', methods=['POST'])
def upload_file():
    logger.info('Upload file request received.')
    # Get the video file from the request
    form_data = request.form
    logger.info('Form data: {}'.format(form_data))
    file = request.files['file']
    logger.info('File: {}'.format(file))
    return 'OK'


if __name__ == '__main__':
    app.run()

Code that I tried:
C# .NET:

// JSON data
var jsonData = new
{
    filename = "test_file.mkv",
    campaign_name = "Test Project Name",
    session_id = "12345678-1234-1234-1234-123456789123",
    activity_id = "12345678-1234-1234-1234-123456789123",
    tags = new List<string> { "video" },
    machine_name = "Test machine name",
    creation_date = "2021-01-01"
};

using (var httpClient = new HttpClient())
{
    if (headers != null)
    {
        foreach (var header in headers)
        {
            httpClient.DefaultRequestHeaders.Add(header.Key, header.Value);
        }
    }

    using (var multipartContent = new MultipartFormDataContent())
    {
        // Add the file
        var fileStream = File.OpenRead(local_path_for_tests);
        var fileContent = new StreamContent(fileStream);
        multipartContent.Add(fileContent, "file", Path.GetFileName(local_path_for_tests));

        // Add the JSON data
        var jsonContent = new StringContent(JsonConvert.SerializeObject(jsonData));
        jsonContent.Headers.ContentType = new MediaTypeHeaderValue("application/json");
        multipartContent.Add(jsonContent, "json");

        // Send the request
        using (var response = await httpClient.PostAsync(server_url, multipartContent))
        {
            response.EnsureSuccessStatusCode();
            string responseBody = await response.Content.ReadAsStringAsync();
            Console.WriteLine(responseBody);
        }
    }
}

Error:

Exception has occurred: CLR/System.Net.Http.HttpRequestException
An exception of type 'System.Net.Http.HttpRequestException' occurred in System.Net.Http.dll but was not handled in user code: 'Response status code does not indicate success: 415 (UNSUPPORTED MEDIA TYPE).'

Python example 1:

# JSON data
data = {
    'filename': 'test_file.mkv',
    'session_id': session_id,
    'campaign_name': "Test Project Name",
    'activity_id': activity_id,
    'tags': ["video"],
    'machine_name': "Test machine name",
    'creation_date': '2021-01-01'
}

files = {
    'file': open(file_path, 'rb')
}

logger.info(f"Sending request to {url} with data: {data} and file: {files['file'].name}")
start = datetime.now()
response = requests.post(url, data=data, files=files)
logger.info(f"Request time: {datetime.now() - start}")
logger.info(f"Response: {response.text}\n")

Error:

<!doctype html>
<html lang=en>
<title>415 Unsupported Media Type</title>
<h1>Unsupported Media Type</h1>
<p>Did not attempt to load JSON data because the request Content-Type was not &#39;application/json&#39;.</p>

Python example 2:

# JSON data
data = {
    'filename': 'test_file.mkv',
    'session_id': session_id,
    'campaign_name': "Test Project Name",
    'activity_id': activity_id,
    'tags': ["video"],
    'machine_name': "Test machine name",
    'creation_date': '2021-01-01'
}

files = {
    'file': open(file_path, 'rb')
}

headers = {
    'Content-Type': 'application/json'
}

json_data = json.dumps(data)

logger.info(f"Sending request to {url} with data: {data} and file: {files['file'].name}")
start = datetime.now()
response = requests.post(url, headers=headers, data=json_data, files=files)
logger.info(f"Request time: {datetime.now() - start}")
logger.info(f"Response: {response.text}\n")

Error:

ValueError: Data must not be a string.