T10285: Set Content-Disposition header on static.miraheze.org requests containing ?download (#3121)

This pull request sets Content-Disposition: attachment on requests at
static.miraheze.org that contain ?download.

This is required by Extension:MultiMediaViewer in order to actually
download the file: see https://phabricator.miraheze.org/T10285.

The code is from Gilles <gdubuc@wikimedia.org>
(https://gerrit.wikimedia.org/r/c/operations/puppet/+/120617), I just
ported it to Miraheze.