docs: def get_files_for_document(document):
      """
      Returns the available files for all languages.
  
      In case the file is already present in another language, it does not re-add
      it again.
  
      """
      files = []
      for doc_trans in document.translations.all():
          if doc_trans.filer_file is not None and \
                  doc_trans.filer_file not in files:
              doc_trans.filer_file.language = doc_trans.language_code
              files.append(doc_trans.filer_file)
      return files