There are already ways to force browsers to download using HTTP headers. The author says this is a "server-side solutions to front-end problem", okay. But anyways, when pages use this already, it OFTEN pisses me off, when I want to look at a PDF in browser, but the web app/page is forcing me to save it to disk and then re-open it in another app. What we _really_ need, that we _don't_ have a way to do right now -- is…
Lends itself reasonably well to a JS-only implementation, too:
http://jsfiddle.net/FLa8P/1/