Apache Poi Format or extension is not valid and file in use locked for editing by another user

Question

I am using Apache Poi to create an Excel file for my project, but I met a few weird issues while trying it.

public void createfile(Request request, Response response) throws Exception {
try {
        Integer id = ParamUtils.getParamInt(request, "id", null);
        Workbook workbook = new HSSFWorkbook();
        Sheet sheet = workbook.createSheet("new sheet");
        FileOutputStream fileOut = new FileOutputStream("workbook.xls");
        workbook.write(fileOut);
        fileOut.close();
        }
        catch (IncorrectData e) {
            return e;
        }

    }

Firstly, I got a format or extension that is not valid error when I try to open the file I create with this function. Secondly, I got file in use locked for editing by another user but I clearly closed OutputStream at the end of the code. One last question, it was a function at my server and I was wondering if I can pass/return this Excel file to the front to download it?

Thanks!