add_action('admin_init', 'redirect_after_media_save');
function redirect_after_media_save() {

    global $pagenow;

    // when visiting the upload screen, we save the number of attachments
    if ( $pagenow == 'media-new.php' ) {
        $attachments_before = array_sum((array)wp_count_attachments());
        update_option('count_attach_before', $attachments_before);
    }

    if ( $pagenow == 'upload.php' ) { // we are on media library page

        // get attachments count before and after upload
        $attachments_before = get_option('count_attach_before');
        $attachments_after = array_sum((array)wp_count_attachments());

        if (
            // there are new files uploaded
            ( wp_get_referer() == admin_url('media-new.php') && $attachments_after > $attachments_before )
        ) {
                // send admin email
                wp_mail( $to, $subject, $message, $headers, $attachments );
        }
    }
}