opendialog1.InitialDir := extractfilepath(application.ExeName); 
savedialog1.InitialDir := extractfilepath(application.ExeName);

procedure TForm1.Button5Click(Sender: TObject);
begin
SaveDialog1.Filter := '*.txt|*.txt'; //设置过滤器，只保存txt
if savedialog1.execute then
     begin
       if extractfileExt(savedialog1.FileName)<>'.txt' then //判断文件名后缀是否为.txt
       SaveDialog1.FileName := ChangeFileExt(SaveDialog1.FileName,'.txt'); //不是的话修改后缀为.txt
 
        Memo2.Lines.SaveToFile(savedialog1.FileName); //保存文件
     end;
end;