I run into that missing peace aswell... Extjs could easily manage that by adding a new Ext type. Like we allready have for formrequests. Which uses form submit to setup a request to the server in the same way an form request does when uploading a file. Bypass the request into an Iframe where the response could be a file now!
I think this would be really great since you could really have one bottleneck http handler to handle all kinds of Communications between Extjs and Serverside!!!