Have ?download parameter trigger Content-Disposition: attachment

Have ?download parameter trigger Content-Disposition: attachment

This parameter triggers Content-Disposition: Attachment which
makes the browser download the image instead of displaying it.

This is needed by Media Viewer to allow users to click a button in
order to download an image at a given resolution or the original.

Change-Id: I470a24a09139ac65588312104995e34d97a89b0f