Mail body corrupted in HTML format

I am not sure if this is a duplicate of #1044534. After more debugging I have found out that my problem was caused by

function mimemail_extract_files($html) {
  $pattern = '/(<link[^>]+href=[\'"]?|<object[^>]+codebase=[\'"]?|@import |[\s]src=[\'"]?)([^\'>"]+)([\'"]?)/mis';
  $content = preg_replace_callback($pattern, '_mimemail_replace_files', $html);
  $encoding = '8Bit';
  /* Quick fix
  $body = explode("\n", $content);
  foreach ($body as $line) {
    if (drupal_strlen($line) > 998) {
      $encoding = 'base64';
      break;
    }
  }
  if ($encoding == 'base64') {
    $content = rtrim(chunk_split(base64_encode($content)));
  }
  */
  $document = array(array(
    'Content-Type' => "text/html; charset=utf-8",
    'Content-Transfer-Encoding' => $encoding,
    'content' => $content,
  ));
  $files = _mimemail_file();
  return array_merge($document, $files);
}

located in mimemail.inc.
My test mail indeed had one very long line, but I don't get a point why to convert all content to base64 encoding? With the commented part as above it works fine, but that's just a quick fix. I hope someone can explain it and fix properly.