> AGO has a new attachment viewer web app, but it is not available as a default option with enterprise 10.7 ... 10.8 update would capture this
You're correct, this configurable app template was added to ArcGIS Enterprise 10.8.
(see Configurable app templates section)
> What is the best way to get new releases of web applications on an on premises enterprise deployment? ... Do I download the app fro AGO and host it
Downloading and added new app templates into Portal for ArcGIS is not a supported workflow. Sorry. Upgrading to the next ArcGIS Enterprise release is how you get new app releases.
Hope this helps,