private function mem_check($props)
    {
        // image size is unknown, we can't calculate required memory
        if (!$props['width']) {
            return true;
        }

        // channels: CMYK - 4, RGB - 3
        $multip = ($props['channels'] ?: 3) + 1;

        // calculate image size in memory (in bytes)
        $size = $props['width'] * $props['height'] * $multip;
        return rcube_utils::mem_check($size);
    }