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function _save_attachment_url($post, $attachment) {
        if ( isset($attachment['url']) ) 
                update_post_meta( $post['ID'], '_wp_attachment_url', esc_url_raw($attachment['url']) ); 
        return $post;
}
add_filter('attachment_fields_to_save', '_save_attachment_url', 10, 2);
function _replace_attachment_url($form_fields, $post) {
        if ( isset($form_fields['url']['html']) ) {
                $url = get_post_meta( $post->ID, '_wp_attachment_url', true );
                if ( ! empty($url) )
                        $form_fields['url']['html'] = preg_replace( "/value='.*?'/", "value='$url'", $form_fields['url']['html'] );
        }
        return $form_fields;
}
add_filter('attachment_fields_to_edit', '_replace_attachment_url', 10, 2);