Change POST data to JSON string and mask data.

def _get_request_param(self, request):
        """Change POST data to JSON string and mask data."""
        params = {}
        try:
            params = request.POST.copy()
            if not params:
                params = json.loads(request.body)
        except Exception:
            pass
        for key in params:
            # replace a value to a masked characters
            if key in self.mask_fields:
                params[key] = '*' * 8

        # when a file uploaded (E.g create image)
        files = request.FILES.values()
        if list(files):
            filenames = ', '.join(
                [up_file.name for up_file in files])
            params['file_name'] = filenames

        try:
            return json.dumps(params, ensure_ascii=False)
        except Exception:
            return 'Unserializable Object'