Depending on how you have Windows preferences set, Windows will suppress showing the file extension of known file types. It appears to know the .tax2016 and .tax2017 file types because those two are listed as file type "TurboTax 201x Document" while the earlier years have the file type as "TAX201x File" (just the extension followed by the word File). Note that with the 2016 and 2017, Windows knows these are TurboTax files while for the earlier years, it doesn't. So almost assuredly, those two have the extension .tax201x but the display of it is suppressed because they're known filetypes. The best thing would be for the poster to install TT2018 and see if it that 2017 file shows up as one that can be imported to start a 2018 return.
