Along with being able to define "kind"s, it would be useful to me to be able to assign more than one extension to the “text” kind within DT. For example, I have some files that are text files with the extension “.mdown” (for “Markdown”, a simple mark-up language). It would be convenient to be able to import an .mdown file directly and still be able to view its text within DT, rather than have it be an opaque file I have to open in TextMate (the application currently associated with .mdown files). Being able to define .mdown as being of kind “text” within DT seems like the simplest approach (plus that would let me “Open with” TextMate for bigger editing jobs, while keeping them in DT for small edits and searching).
I can work around it, of course, either with copy and paste or via bulk renames to change file extensions, but I don’t want to.