Add "not available" label on unknown media attachments (#11715)

app/javascript/styles/mastodon/components.scss

@@ -3477,6 +3477,14 @@ a.status-card.compact:hover {
         background: rgba($base-overlay-background, 0.8);
       }
     }
+
+    &:disabled {
+      cursor: not-allowed;
+
+      .spoiler-button__overlay__label {
+        background: rgba($base-overlay-background, 0.5);
+      }
+    }
   }
 }