Beautiful find, Dmitri!

As of 4.4, this has indeed been added:
https://developer.wordpress.org/reference/hooks/wp_theme_editor_filetypes/

But from the look of the WP Core code:
https://core.trac.wordpress.org/browser/tags/4.6/src/wp-admin/theme-editor.php#L76
It looks like you might want to do something more like this, no?

add_filter(‘wp_theme_editor_filetypes’, function ($types) {
    $types[] = ‘scss’;
    $types[] = ’js’;
    return $types;
});