Add download query parameter to download images as attachment.

lib/units/storage/plugins/image/index.js

@@ -31,6 +31,12 @@ module.exports = function(options) {
         })
         .then(function(out) {
           res.status(200)
+
+          if (typeof req.query.download !== 'undefined') {
+            res.set('Content-Disposition',
+              'attachment; filename="' + req.params['0'] + '"')
+          }
+
           out.pipe(res)
         })
         .catch(function(err) {