ah, that's a good point.

perhaps use .ll.jxl to indicate "lossless jpegxl" informally?

prior art: I've been using .frontmatter.md or .fm.md for markdown files with frontmatter

I've been wishing for years that all of these codecs with hybrid lossless/lossy modes would just add a single l to their file extensions, purely for the annoyance of it.

Like, you can do this individually, but it's not the same thing as actually having it in the standard.

Multiple extensions typically denote nested filetypes.

Like files.tar.xz may be decompressed with xz and then extracted with tar to get the directory "files".

I think in your case .md.fm would be a greater fit, as the front matter is read first. Or just .fmd