by accepting "MS Excel 2007 XML" in isSpreadSheetFile().

Test example:

Workbooks.Open Filename:="/tmp/test.xlsx"

Change-Id: Id7102325c96842bb413b1fb7bb8162e8bca41f4e
Reviewed-on: https://gerrit.libreoffice.org/57030Reviewed-by: László Németh <nemeth@numbertext.org>
Tested-by: László Németh <nemeth@numbertext.org>