Comment 41 for bug 25830

Everyone agrees this would be good. This at this point requires architecture
changes to the URILoader -- once we determine that we have no content viewer for
the data we seeem to pass it off to the external helper app handler and have no
way of getting it back into core layout again even if the external helper app
handler jumps through hoops.

I'm working on some stuff in there this weekend.... if I fix it, good. If not,
I would strongly recommend passing on this for 1.0 -- this is the sort of arch
change that should bake on the trunk and in milestones for a while.