Field template in theme based on Garland
Your preprocess function is not necessary in this situation because Drupal's default behavior is using this sort of files. Try print_r($variables['theme_hook_suggestions']); and see all relevant suggestions.

Your problem is the wrong filename; try field--field_is_new.tpl.php instead.

The general recommendation is using theme functions:

function theme_field__field_taxonomy($vars) {
  $output = '';

  foreach ($vars['items'] as $delta =&gt; $item) {
    $array = explode(';',$item['#markup']);

    if (count($array)) {
      $output = '&lt;ul class="' . $vars['classes'] . ' clearfix"' . $vars['attributes'] . '&gt;';
      foreach ($array as $value) {
          $output .= '&lt;li&gt;' . trim($value) . '&lt;/li&gt;';
      }
      $output .= '&lt;/ul&gt;';
    };
  }

  return $output;
}