protected function isValidZip( $fullpath ) {

		$result = $this->zip->open( $fullpath );

		if ( true !== $result ) {
			throw new \Exception( 'Opening docx file failed' );
		}
		// check if a document.xml exists
		$path = $this->getTargetPath( self::DOCUMENT_SCHEMA );

		$ok = $this->getZipContent( $path );

		if ( ! $ok ) {
			throw new \Exception( 'Bad or corrupted _rels/.rels' );
		}
	}