It supports multiple extensions just fine. I use that every day (with md markdown meaning blah.md and blah.markdown both get markdown highlighting). And clj cljs would be the right syntax for doing multiple extensions.

The UDL extension recognition only applies when you first create or open a file – if you already had the blah.cljs file open as “Normal text” when you edited the UDL, you would have to close the file and reopen (Ctrl+W, Ctrl+Shift+T), at which point it should see that it has the right extension, and use the UDL syntax highlighting. If that’s not it, you’ll have to give a bit more info.