Dimensions are saving as .xls instead of .xlsx

I am on BPC 7.0. My dimensions are currently saving as .xls (Excel 2003) files when I process the dimesnion from Admin. How can I change this so that the dimensions will save as .xlsx (Excel 2007) files? I have tried opening the dimension file in Excel 2007, then saving it as an .xlsx file, but when I try to open the dimension from Admin afterward, it does not recognize the file.

Any help would be appreciated. Thanks.