docs: @Nonnull
    protected File getTempFile ()
    {
      if (m_aTempFile == null)
      {
        // If you manage to get more than 100 million of ids, you'll
        // start getting ids longer than 8 characters.
        final String sUniqueID = StringHelper.getLeadingZero (s_aTempFileCounter.getAndIncrement (), 8);
        final String sTempFileName = "upload_" + UID + "_" + sUniqueID + ".tmp";
        m_aTempFile = new File (m_aTempDir, sTempFileName);
      }
      return m_aTempFile;
    }