docs: public static String getDocbookFileName(final RESTImageV1 source) {
          checkArgument(source != null, "The source parameter can not be null");
  
          if (source.getLanguageImages_OTM() != null && source.getLanguageImages_OTM().getItems() != null) {
              final List<RESTLanguageImageV1> langImages = source.getLanguageImages_OTM().returnItems();
              for (final RESTLanguageImageV1 langImage : langImages) {
                  final String filename = langImage.getFilename();
                  if (filename != null) {
                      final int indexOfExtension = filename.lastIndexOf('.');
  
                      if (indexOfExtension != -1 && indexOfExtension < filename.length() - 1) {
                          final String extension = filename.substring(indexOfExtension, filename.length());
                          return source.getId() + extension;
                      }
                  }
              }
          }
  
          return "";
      }