List of compatible types is way more than that, according to the Windows-specific upload dialog -- includes eyebrow-raisers like BMP, Windows Icon format, XCF (GIMP iirc), HEIC (x265 has a static image implementation?), Adobe Illustrator files, among others.
If none of those are an option on lemmy.world or other instances, guess that at least proves that Pawb's fork of the Lemmy code can accept other formats.
Big caveat to start, I'm not an application programmer. I don't know how the web browser communicates with the operating system for the open dialog. Its possible those types are supported, or its possible the browser just tells windows, Open up a file selector and valid file types are images and videos. What those file types are is left up to the operating system.
For a quick search, I'm not sure either firefox or chrome support jxl.