I used this install extension in Alfred 1 a fair amount, but the developer doesn't seem to have posted anything for a long time, I doubt it will be updated for Alfred 2. Someone want to tinker with it? It takes a downloaded file (zip, mpkg, pkg, dmg, and originally alfredextension files) and installs them, then moves the installation file to the trash.
Might be useful with all the new workflows flying around!
Original - http://otijhuis.tumblr.com/post/12988763001/alfred-install-extension