[CaRDiaK]

I create a new file in notepad and save it as simple.csv with exactly this content and I get "Invalid File. Please upload only csv, tsv files."

[nebabyte]

Also be sure to switch it off of "plain text file" or it'll save as file.csv.txt (though if you have 'hide common file extensions' off you'd've noticed this in your file browser)

iirc notepad auto prepends .txt anyways if that value isn't changed.

[Freak_NL]

Weird. I'm on Linux though, but I can't imagine that the line endings should matter.