respond_to for unknown type


How would I go about handling an unknown type in a respond_to block?

That is, I currently have ...

respond_to do | format |
  format.html { do something }
  format.xml { do something else }

I would like a handler for all other format's, other than html/xml, so
that I can, for example, log the request, render an error or whatnot.

Thank you,