Varnish: add ?download parameter to upload

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.

Change-Id: I73198f4f294ee36e093dd135e2c38ac604559bb3
Mingle: https://wikimedia.mingle.thoughtworks.com/projects/multimedia/cards/385
Merge "Varnish: add ?download parameter to upload" into production