Fix small image previews in the file viewer.

See also https://gitlab.com/gitlab-org/gitlab-ce/issues/20505#note_13670085

app/assets/stylesheets/framework/files.scss

@@ -63,9 +63,10 @@
     &.image_file {
       background: #eee;
       text-align: center;
+      
       img {
-        padding: 100px;
-        max-width: 50%;
+        padding: 20px;
+        max-width: 100%;
       }
     }