Backdrop CMS: Open PDF and TXT files in new window

template.php

<?php

function MYTHEME_file_link($variables) {
  $file = $variables['file'];
  $icon_directory = $variables['icon_directory'];

  $url = file_create_url($file->uri);
  $icon = theme('file_icon', array('file' => ($file instanceof File) ? $file : file_load($file->fid), 'icon_directory' => $icon_directory));

  // Set options as per anchor format described at
  // http://microformats.org/wiki/file-format-examples
  $options = array(
      'attributes' => array(
          'type' => $file->filemime . '; length=' . $file->filesize,
      ),
  );

  // Use the description as the link text if available.
  if (empty($file->description)) {
    $link_text = $file->filename;
  } else {
    $link_text = $file->description;
    $options['attributes']['title'] = check_plain($file->filename);
  }

  //open files of particular mime types in new window
  $new_window_mimetypes = array('application/pdf', 'text/plain');
  if (in_array($file->filemime, $new_window_mimetypes)) {
    $options['attributes']['target'] = '_blank';
  }
  
  return '<span class="file">' . $icon . ' ' . l($link_text, $url, $options) . '</span>';

}