code: public final boolean isFileAllowedForPreprocessing(@Nullable final File file) {
      boolean result = false;
      if (file != null && file.isFile() && file.length() != 0L) {
        result = this.extensions.contains(PreprocessorUtils.getFileExtension(file));
      }
      return result;
    }
docs: Check that a file is allowed to be preprocessed fo its extension
  
  @param file a file to be checked
  @return true if the file is allowed, false otherwise