How to post a file with chinese characters in TIdHttp

Question

I'm using telegram bot api to send a document. it was working OK until I need to send files with chinese characters. the file is sent correctly but the filename is garbage. Here is my code with tests :

procedure TSendThread.SendTelegramMessage(filename, chat_id, aCaption: string);
const
  c_url = 'https://api.telegram.org/bot893812066:YOUR_TOKEN/sendDocument';
var
  idHttpEx : TIdHTTPEx; // this is good as TIdHttp
  aParam : TIdMultipartFormDataStream;
begin


  idHttpEx := TIdHTTPEx.Create(nil);
  aParam  := TIdMultipartFormDataStream.Create;
  try

    aParam.AddFormField('chat_id', chat_id);
    aParam.AddFormField('caption', aCaption, 'utf-8').ContentTransfer := '8bit';
    aParam.AddFile('document', filename); //original code

    // test 1
//    with aParam.AddFile('document', filename) do
//    begin
//      Charset := 'utf-8';
//      ContentTransfer := '8bit';
//    end;

      // test 2
//    aParam.AddFile('document', filename).Charset := 'utf-8';

      // test 3
//    aParam.AddFile('document', filename, 'utf-8').ContentTransfer := '8bit';

      // test 4
//    aParam.AddFile('document', UTF8Encode(filename)).ContentTransfer := '8bit';

    idHttpEx.Post(c_url, aParam);

  finally
    aParam.Free;
    idHttpEx.Free;
  end;
end;

How do I post files with chinese filename correctly? Where did I go wrong?

*****EDIT***** The filename is chinese like : 强大.txt. I still receive the file intact but the filename is now : = UTF-8 B 5paw5LiW55WMXzIwMTktMDYtMjUuVFhU = which is incorrect.