Dex

procedure TForm1.Button1Click(Sender: TObject);
var
  Stream: TFileStream;
begin
  if SaveDialog1.Execute then begin
    if UpperCase(ExtractFileExt(SaveDialog1.FileName)) = '.TXT' then begin
      Stream := TFileStream.Create(SaveDialog1.FileName, fmCreate);
      try
        Stream.Write(Richedit1.Lines.Text[1], Length(Richedit1.Lines.Text));
      finally
        Stream.Free;
      end;
    end else RichEdit1.Lines.SaveToFile(SaveDialog1.FileName);
  end;
end;