It's a fairly new format, only standardized this year (2022). It has some very useful features though, not least of which is being able to losslessly transcode JPEG files with around 20% smaller output.
This initial beta release needs more extensive testing. It is rather slow and I am looking at ways to optimize it. ICC colour profiles are not supported either, but it's not clear exactly how the decoding works so I'd be interested to know if the colour looks right on files with those profiles.
Finally, HDR is not supported either. I'm not sure if it can be supported, haven't really looked.
Copy both the .dll and .lld into the Viewers folder and refresh the plugin list in Opus.
I'll get an updated version posted soon. In the mean time, I think the issue is you need to have the DLL in the main DOpus program folder, not the plugins folder. I'm not sure why, it is supposed to look in both places. When I have time I'll debug it.
Yeah, I mean the libjxl.dll, not the plugin. I wrote code that is supposed to check the current directory and then the current directory + "viewers/" or whatever the path is, but it clearly doesn't work. I'm not really a desktop developer, I do embedded, so bare with me or feel free to submit pull requests.
Things stalled a bit because libjxl has performance issues, and they broke the Windows build too. At some point I will try to cross compile it from WSL and update.