Perhaps `file_ext_lower` would be a clearer name for it ...

Otherwise I'm commenting here because you need to follow the [styleguide](http://ellislab.com/codeigniter/user-guide/general/styleguide.html) ... there are too many details in here, so I'll just show you how the end solution should look like with some improvements from me:

    if (count($x) === 1)
    {
        return '';
    }

    $ext = ($this->file_ext_case) ? strtolower(end($x)) : end($x);
    return '.'.$ext;

You should also remove the empty line at EOF.