For SSRS 2016 in data driven subscription for file share and enter value of Excel for Render Format and it creates an .xlsx file. If I use a value from the dataset, which is returned as "Excel" the created file is .xls instead, and about twice as big.
I could not find anything in ReportSever tables or views that showed exactly how the subscription was interpreted.
I would expect that the value "Excel" when pulled from the data driven query would behave the same way (create .xlsx file) as selecting "Excel" as entered value.