Well, STIF is not a distinct type, but PCI uses STIF and I chose to class it
as Content-type:Text on the theory that humans ought to be quite comforable
reading the stuff. (That was, after all, a design goal for STIF.)
Sounds like you made the right choice.
The spec for Signature uses Application.
I haven't read the signature specification in any detail, but offhand I'd
say this is one subtype that needs to be under text! And if there's some
reason the data itself isn't sufficiently text-like, I'd say that its
the data format that needs to change rather than the grouping!
Those are the two that pop to mind immediatley. If I foraged, I'd probably
come up with more. Lots of conversations have also triggered this question.
A quick glance over the current IANA registry shows no subtypes of application
that I could see belonging under text, plus one text subtype not in the base
MIME specification which nicely illustrates that this point isn't something
registrars have overlooked.