Default Excel format: XLSX instead of XLS

[Manuel Luque]

I think the default format for Excel spreadsheets should be XLSX instead of XLS. I think we might stopping providing support to XLS format, as it is very old (XLSX was included in Excel 2007, almost 16 years ago) and it was never an ISO standard. XLSX format belongs to the specification ISO/IEC 29500-1:2016.

[Manuel Luque]

We have decided to make this change.