So the browser would be responsible for:
AFAIK it already knows how to do the first and third, and the second is just a matter of using what 99% of people already use today. Some judicious use of temp directories (which they're already doing, right?) and you're set.
This doesn't seem very difficult and I'm sure I'm not the first to think of it. Is there something like this now? (Besides PDF or PowerPoint...)