fails:File.extname returns the extension (the portion of file name in path after the period).
fails:File.extname returns the extension (the portion of file name in path after the period).(edge cases)
fails:File.extname returns only the last extension of a file with several dots
fails:File.extname accepts an object that has a #to_path method
fails:File.extname raises a TypeError if not passed a String type
fails:File.extname raises an ArgumentError if not passed one argument
fails:File.extname returns the extension for a multibyte filename
