Register WordPress tags for posts of the attachment post type, register other custom taxonomies as well

tags_for_attachments.php

<?php
/**
 * Register WordPress tags for posts of the attachment post type, register other custom taxonomies as well
 */
function sjf_deh_taxonomies() {
	
	// allow tags to apply to posts of the attachment post_type
	register_taxonomy_for_object_type( 'post_tag', 'attachment' );

	// Add new taxonomy, make it hierarchical (like categories)
	$labels = array(
		'name'              => _x( 'Skiers', 'taxonomy general name' ),
		'singular_name'     => _x( 'Skier', 'taxonomy singular name' ),
		'search_items'      => __( 'Search Skiers' ),
		'all_items'         => __( 'All Skiers' ),
		'parent_item'       => __( 'Parent Skier' ),
		'parent_item_colon' => __( 'Parent Skier:' ),
		'edit_item'         => __( 'Edit Skier' ),
		'update_item'       => __( 'Update Skier' ),
		'add_new_item'      => __( 'Add New Skier' ),
		'new_item_name'     => __( 'New Skier Name' ),
		'menu_name'         => __( 'Skiers' ),
	);

	$args = array(
		'hierarchical'      => false,
		'labels'            => $labels,
		'show_ui'           => true,
		'show_admin_column' => true,
		'query_var'         => true,
		'rewrite'           => array( 'slug' => 'skiers' ),
	);

	register_taxonomy( 'skier', array( 'attachment' ), $args );

}
add_action( 'init', 'sjf_deh_taxonomies', 0 );