Hi Roberto,
It is not looking so simple.
We have Office 2007 on the Application server and I tried creating a test dimension in admin module but it creates the dimension file in .xls format and then I changed the extension of test dimension from .xls to .xlsx and try to open the test dimension from admin module then it displays message that "Dimension member sheet not found"
There might be some internal settings and that is creating new file in .xls format and recognizing the .xls format file when opening.
Thanks & Regards,
Rohit