In my case:

[curt:misc]$ mdls -name kMDItemContentTypeTree U.scala
kMDItemContentTypeTree = (
    "dyn.ah62d4rv4ge81g25brvuu",
    "public.item",
    "dyn.ah62d4rv4ge81g25brvuu",
    "public.data"
)

public.source-code is not in the list.  How can I add it to the list of Content Types?