Media taxonomy that Relevanssi can index

attachment-taxonomy.php

<?php

// Add this code to theme functions.php

add_action( 'init', 'create_media_tax' );

function create_media_tax() {
	register_taxonomy(
		'media_tag',
		'attachment',
		array(
			'label' => __( 'Media tag' ),
			'rewrite' => array( 'slug' => 'media_tag' ),
			'hierarchical' => true,
		)
	);
}

?>