What is XLSX?
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.
What is SQL?
A table as SQL is a CREATE TABLE and a run of INSERT statements: the form data takes when it is going into a database rather than a spreadsheet. Dialects differ on quoting, so this is written to the ANSI standard and says so in the file's first line.