UI: Scale the attachemnt thumbnails

The attachements should be scaled inside the card, too.

client/components/cards/attachments.styl

@@ -32,8 +32,8 @@
       position: relative
 
       .attachment-thumbnail-img
-        height: 100%
-        width: 100%
+        max-height: 100%
+        max-width: 100%
 
       .attachment-thumbnail-ext
         text-transform: uppercase