Get all media categories

I’ve added a new taxonomy for the medias post type:

function register_taxonomy_for_images()
{
    register_taxonomy_for_object_type('category', 'attachment');
}

add_action('init', 'register_taxonomy_for_images');

And I’ve added a few categories via wordpress admin.How can I get an array of all the media categories that I added? I’ve tried get_categories() but it doesn’t work.