Custom taxonomy not showing in grid view for ‘attachment’ post type

Setting the argument public to true will do. Check out the file wp-admin/includes/media.php, function get_compat_media_markup() for reference:

if ( $args['in_modal'] ) {
    foreach ( get_attachment_taxonomies( $post ) as $taxonomy ) {
        $t = (array) get_taxonomy( $taxonomy );

        if ( ! $t['public'] || ! $t['show_ui'] ) {
            continue;
        }