XLSX is what Excel has written since 2007 - not one file but a zipped folder of XML, with the sheet in one part and the strings in another. Its famous trap is dates: a date is stored as a plain number of days, and only the formatting attached to the cell says it is a date rather than a quantity.
Every conversion below runs in this page, on your own machine. Nothing is uploaded, there is no queue, and there is no account to make. Pick a target format, or just drop a XLSX file above and choose afterwards.