I assumed GP was referring to the filename extension. Of course people put all sorts of nonsense into files and then call them ".csv", but it's not a bad heuris

I assumed GP was referring to the filename extension. Of course people put all sorts of nonsense into files and then call them ".csv", but it's not a bad heuristic to use that to guess the format as a default, and even guess that the output format should be the same, in the absence of other flags.