docs: @Override
      public AttachedFile[] getFiles(String CorpNum, MgtKeyType KeyType,
                                     String MgtKey) throws PopbillException {
          if (KeyType == null)
              throw new PopbillException(-99999999, "관리번호형태가 입력되지 않았습니다.");
          if (MgtKey == null || MgtKey.isEmpty())
              throw new PopbillException(-99999999, "관리번호가 입력되지 않았습니다.");
  
          return httpget("/Taxinvoice/" + KeyType.name() + "/" + MgtKey
                  + "/Files", CorpNum, null, AttachedFile[].class);
      }