Using MYTHEME_page_attachment_alter does not alter the favicon
If its not a browser cache issue perhaps you should try modifying it with a hook called later namely THEMENAME_preprocess_html

function THEMENAME_preprocess_html(array &amp;$variables) {
  $test = "";
  $variables["page"]["#attached"]["html_head_link"][0][0] =
    array(
      'rel' =&gt; 'shortcut icon',
      'href' =&gt; '/themes/fu/favicon_urbana.png',
      'type' =&gt; 'image/png',
    );
}