Hi,
The `mediaViewer` feature will only work for Items that have files that have a media type (mimetype) starting with "image/" or "video/". So, "image/gif" would be supported, as would "video/mpg". But, you would just see the regular thumbnails for all other items.
Also, make sure you are modifying the "config.prod.yml" file if you are running in production mode... similarly modify "config.dev.yml" if you are using dev mode. You haven't clarified how you are building/running the UI.
If you need more help, let us know on this list.
Tim