public function addExtension($extension)
    {
        $extensions = $this->getExtension();
        if (is_string($extension)) {
            $extension = explode(',', $extension);
        }

        foreach ($extension as $content) {
            if (empty($content) || !is_string($content)) {
                continue;
            }

            $extensions[] = trim($content);
        }
        $extensions = array_unique($extensions);

        // Sanity check to ensure no empty values
        foreach ($extensions as $key => $ext) {
            if (empty($ext)) {
                unset($extensions[$key]);
            }
        }

        $this->_extension = implode(',', $extensions);

        return $this;
    }