Chats: improve display of images, because we can

app/styles/chats.scss

@@ -326,13 +326,17 @@
 }
 
 .chat-message .media-gallery {
-  height: 120px !important;
+  height: auto !important;
   margin: 4px 0 8px;
 
   .spoiler-button {
     display: none;
   }
 
+  &__preview {
+    background-color: transparent;
+  }
+
   &__item-thumbnail img,
   &__item-thumbnail .still-image img {
     object-fit: contain;