Attachments | WordPress.org

Unfortunately, you can’t just make an attachment private. That would be a neat solution. However, you can do this:

add_filter( 'relevanssi_post_ok', function ( $show, $post_id ) {
    if ( ! is_user_logged_in() ) {
        $post = relevanssi_get_post( $post_id );
        if ( 'attachment' === $post->post_type ) {
            $show = false;
        }
    }
    return $show;
}, 11, 2 );

This will exclude the attachment posts from the search for non-logged-in users.

That worked, thanks a lot!