file_styles thumbnail image

The file_styles default image theme function refers to an image in the core files module that does not exist:

function theme_file_styles_styles_default($variables) {
  $file = $variables['object'];
  $path = drupal_get_path('module', 'file');
  $thumbnail = theme('image',
    array(
      'path' => $path . '/images/file-unknown.png', // <-- this really is unknown!
      'alt' => t('Thumbnail for !filename.', array('!filename' => $file->filename)),
      'attributes' => array('width' => 100, 'height' => 75, 'class' => 'file-unknown'),
    )
  );
  return $thumbnail;
}

Ideally this would display different images depending on the file type, and also allow file type images to be run through imagecache so that they fit into a list of files.