Comment by spaceducks

The names and extensions of JPEG XL files aren't specified, except that the IANA media type is `image/jxl`. I think an argument could be made to use the double extension convention when the encoder performs lossless JPEG recompression, so image.jpg becomes image.jpg.jxl (while not entirely semantically correct, since it's not an additional layer of compression around the JPEG, it's a reimplementation of the image using identical coding features as JPEG, in JXL).

But like I said in my other comment (which got hidden for some reason), it should be noted that a recompressed JPEG is also a valid JXL on its own. If you have the means to turn a recompressed JPEG into the original, you also have the means to view native JXLs.

Hopefully adoption is widespread and we won't really have to worry about it. JPEG XL is a much more appealing format than WebP, and unlike WebP there are great arguments for software to support it other than "Google started using them, so they're everywhere now."