public function getPngType($filename = null)
    {
        $filename = $filename ? $filename : $this->pathToImage;

        $pngType = ord(file_get_contents($filename, false, null, 25, 1));

        if ($this->verbose) {
            $this->log("Checking png type of: " . $filename);
            $this->log($this->getPngTypeAsString($pngType));
        }

        return $pngType;
    }