mp4 for all browsers

function yourthemename_preprocess_islandora_video(&$variables) {
  $viewer_dsid = 'MP4';
  $object = $variables['object'];

  $video_params = array(
    'pid' => $object->id,
  );

  if (isset($object[$viewer_dsid]) && islandora_datastream_access(FEDORA_VIEW_OBJECTS, $object[$viewer_dsid])) {
    $video_url = url("islandora/object/{$object->id}/datastream/$viewer_dsid/view");
    $video_params += array(
      'mime' => 'video/mp4',
      'url' => $video_url,
    );
  }
  // Thumbnail.
  if (isset($object['TN']) && islandora_datastream_access(FEDORA_VIEW_OBJECTS, $object['TN'])) {
    $video_params += array(
      'tn' => url("islandora/object/{$object->id}/datastream/TN/view", array('absolute' => TRUE)),
    );
  }

  $viewer = islandora_get_viewer($video_params, 'islandora_video_viewers', $object);
  $variables['islandora_content'] = '';
  if ($viewer) {
    $variables['islandora_content'] = $viewer;
  }
  else {
    $variables['islandora_content'] = NULL;
  }
}