Fix attachment viewer not being hidden

client/components/cards/attachments.css

@@ -1,3 +1,6 @@
+.hidden {
+  display: none;
+}
 .attachment-upload {
   text-align: center;
   font-weight: bold;

client/components/cards/attachments.jade

@@ -31,9 +31,7 @@ template(name="attachmentDeletePopup")
   button.js-confirm.negate.full(type="submit") {{_ 'delete'}}
 
 template(name="attachmentViewer")
-// Disabled attachment viewer, because it opens with empty content
-// when opening card.
-//#viewer-overlay.hidden
+  #viewer-overlay.hidden
     #viewer-top-bar
       span#attachment-name
       a#viewer-close.fa.fa-times-thin