Pretty Attachment Links

pretty-attachment-links.php

<?php
//Plugin Name: Pretty Attachment Links

//change attachment link format
add_filter('attachment_link', 'pal_change_link', 10, 2 );
function pal_change_link( $link, $id ) {
	return home_url("/media/$id/");
}

// Adding a new rule
add_filter('rewrite_rules_array', 'pal_insert_rewrite_rules');
function pal_insert_rewrite_rules( $rules ) {
	$newrules = array();
	$newrules['media/(\d*)$'] = 'index.php?attachment_id=$matches[1]';
	return $newrules + $rules;
}

// flush_rules() if our rules are not yet included
add_action('wp_loaded', 'pal_flush_rules');
function pal_flush_rules() {
	$rules = get_option( 'rewrite_rules' );

	if ( ! isset( $rules['media/(\d*)$'] ) ) {
		global $wp_rewrite;
	   	$wp_rewrite->flush_rules();
	}
}