Should "text/markdown" be "text/vnd.observable.markdown" (support both?)

[GordonSmith]

Since the observable markdown has specific nuances compared to vanilla markdown, would it be sensible to distinguish it as such?

[mbostock]

It feels simpler to use the standard MIME type even if we allow JavaScript interpolation. What would be the advantage of a more specific MIME type? (And presumably that would extend to every thing else, since HTML, SQL, etc., also support JavaScript interpolation.)