When you add a Gallery and bind a Collection built from an Attachments datacard, the images populate in the collection but the names don't.
Watch this 2 minute video which demonstrates this issue when building an app from scratch.
This should be possible, but there is currently a bug with the Attachments control that was introduced a couple of weeks ago. Because of the holidays, the fix won't make production until 2019.
In the meantime the only workaround is creating your app with the older version of the Studio. You can do that by modifying the studio URL like so:
If you are editing an existing app, and the older version of the Studio can't open it, that means that you will first have to roll back the app:
You can now edit your app with 3.18111.17. Note that this version will not be Live (seen by your users) until you "Publish" it. You can see that in the "Published" column of that same app version list.