Uploading images to a Flask API using FlaskClient

I have the following endpoint in my Flask API:

@api.route("/files", methods=Method.POST)
def upload_file() -> str:
    enquirer_id = request.get_json()['id']

    try:
        logging.info(request.files)
        uploaded_file = request.files['image']
    except KeyError:
        abort(HttpStatusCode.BAD_REQUEST, "Please provide image to upload")

    if uploaded_file.content_type != ContentType.JPEG:
        abort(HttpStatusCode.BAD_REQUEST, "Only JPEG images are allowed")

    filename = str(enquirer_id)
    destination_file = os.path.join(current_app.config[AppCfg.DIRECTORY], filename)
    uploaded_file.save(destination_file)

    return f'Successfully upload file {filename}.'

But I am unable to test this endpoint using the FlaskClient:

class APITestUploadDownload(APITestBase):
    url = '/api/v1/files'
    my_file = 'some_picture.jpg'

    def setUp(self):
        self.client = self.app.test_client()

    def test_upload(self):
        with open(file=self.my_file, mode='rb') as file:
            data = {
                'image': io.BytesIO(file.read()),
                'id': 1
            }
            response = self.client.post(self.url, data=json.dumps(data), headers={}, content_type='multipart/form-data',)

        self.assertEqual(response.status_code, HttpStatusCode.OK)

But this fails with the error message TypeError: Object of type BytesIO is not JSON serializable.

If i make the request without using json.dump(data) using response = self.client.post(self.url, data=data, headers={}, content_type='multipart/form-data', ) I get another error which tells me thatresponse.data is None.

So how can I fix my file upload endpoint and the correspondig test?