Interesting.  So we have two choices: leave it to the platform mime types file to define because it is not even on track to be an official IANA standard, or include it with a comment that it is a de-facto standard.

The question, I guess, is how fluid the definition is likely to be.  It doesn't seem like the filetype mapping for the existing definitions is going to change, and since we don't (currently) have content-detection of any sort, the impact of a change in the definitions to users of our code seems minimal.  So I guess I'd be OK with adding it as a de-facto standard, though I'm not entirely comfortable with it.  But that would represent a change in policy, so others may want to weigh in.