public function getFileUrl($entity, $field, $fileName = null)
    {
        if (func_num_args() < 3) {
            if ($entity) {
                $fileName = PropertyHelper::getValue($entity, $field);
            }
        }

        if ($fileName instanceof File) {
            $fileName = $fileName->getBasename();
        }

        if ($fileName) {
            return ltrim($this->httpRoot . '/' . $this->getRelativePath($entity, $field) . '/' . $fileName, '/');
        }

        return null;
    }