Java POI the supplied data appears to be in the Office 2007+ XML

According to the Apache POI Quick Guide, the POIFSFileSystem (or similarly, NPOIFSFileSystem) is only used with .xls (Excel versions through 2003) documents.

The equivalent for .xlsx documents (Excel 2007+) is OPCPackage.

OPCPackage pkg = OPCPackage.open(new File("file.xlsx"));

You can create an XSSFWorkbook from the OPCPackage:

XSSFWorkbook wb = new XSSFWorkbook(pkg);

Or you can just create it directly:

XSSFWorkbook wb = new XSSFWorkbook(new File("file.xlsx"));

Generally it’s better to create the workbook using a File instead of an InputStream, to save memory.

Also, if you want code that doesn’t care whether it’s an .xls or an .xlsx:

// or "file.xlsx"
Workbook wb = WorkbookFactory.create(new File("MyExcel.xls"));