Add categories to media (attachments) in wordpress

functions.php

...

add_action( 'init', 'create_media_category' );

function create_media_category() {
	register_taxonomy(
		'media_category', // taxonomy machine name
		'attachment', // content type
		array(
			'labels' => array(
				'name' => __( 'Media Categories' ),
				'singular_name' => __( 'Media Category' )
			),
			'hierarchical' => true,
		)
	);
}

...