When you export contact info your choices are *.csv and *.xls formats.
MS Excel has used the *.xlsx format since EXCEL 2007. How about updating the formats for EXPORTS?
Such an update would save your customers from having to re-save to the newer (by 10 years) format! <grin> It would also help in visually locating the file as *.xls files have different icons than *.xlsx (at least on my computers (WIN 7 PRO and WIN 10.)