I tried the suggested line and it caused an error so that my behaviours did not finish loading. I altered a similar piece of code from a more recent post and got this...
[:files :lt.objs.files/file-types [{:exts [:mustache], :mime "htmlmixed", :tags [:editor.html], :name "mustache"}]]
...which worked. Pasted in here because this is the top hit on google for: light table mustache