Changing attachment urls?

You can do the following:

/* add new rewrite rule */
function attachment_rewrite( $wp_rewrite ) {
    $rule = array(
        'media/(.+)' => 'index.php?attachment=" . $wp_rewrite->preg_index(1)
    );

    $wp_rewrite->rules = $rule + $wp_rewrite->rules;
}
add_filter( "generate_rewrite_rules', 'attachment_rewrite' );

/* redirect standard wordpress attachments urls to new format */
function redirect_old_attachment() {
    global $wp;

    if( !preg_match( '/^media\/(.*)/', $wp->request ) && isset( $wp->query_vars['attachment'] ) ) {
        wp_redirect( site_url( '/media/' . $wp->query_vars['attachment'] ) , 301 );
    }
}
add_filter( 'template_redirect', 'redirect_old_attachment' );