Additions to extraAllowedFileExtensions not showing up in the checkbox list When adding 'extraAllowedFileExtensions' =&gt; 'css' to /craft/config/general.php it doesn't show up in the list below Restrict allowed file types?. Am I missing something? Is it just not "restrictable" to an added filetype?
It's not "restrictable", because you just added it. Why would you then go back and restrict it?

Using extraAllowedFileExtensions doesn't change the checkbox list at all. It simply allows you to upload files in addition to what is available on that list.