IWorkbook workbook;

            string fileExt = Path.GetExtension(filePath);

            try

            {

                using (var file = new FileStream(filePath, FileMode.Open, FileAccess.Read))

                {

                    if (fileExt == ".xls")

                    {

                        workbook = new HSSFWorkbook(file);

                    }

                    else if (fileExt == ".xlsx")

                    {

                        workbook = new XSSFWorkbook(file);

                    }

                    else

                    {

                        return null;

                    }

                }

            }