How to hook into an attachment?
An attachment is just like any other view (except that it is attached to other views), and so can be manipulated as such: 

In template.php:

function THEMENAME_preprocess_views_view_fields(&amp;$vars) {
  if (isset($vars['view']-&gt;name) &amp;&amp; $vars['view']-&gt;name == 'VIEW_NAME' &amp;&amp; $vars['view']-&gt;current_display == 'ATTACHMENT_NAME') {
    foreach ($vars['fields'] as $id =&gt; $field) {
      if ($id == 'FIELD_NAME') {
        $vars['fields'][$id]-&gt;content = 'new value';
      }
    }
  }
}


Remember to clear your cache.