Is there still no way to show attachments from a related table in a popup with the new map viewer (formerly beta), maybe using Arcade? Also, in the classic web viewer, I was able to view the attachment links with a related table link in the popup, and by showing the tables, but I do not see that option in the new map viewer, am I missing something, see screenshot below? Do I need to do something with Arcade expressions to display the attachment links? Thank you.
As far as I know there is no way to view attachments from a related table in the new map viewer. It does show the number of attachments but doesn't have the ability to show the attachments. I am going to report this as a bug to ESRI technical support because the old map viewer had the ability to show attachments. Hopefully it will be addressed soon and in the next AGO update.
It's possible, the new UI got me as well!
Open the new Web Map Viewer > Layer > Configure Pop-ups.
At the bottom of the Pop-ups configuration pane there is a butt to '+ add content'. This lets you add cool things like more fields, charts, images... and attachments.
Now showing in pop-up:
This has been available since at least 10.8.1 in Portal. Would hope there's been some improvements to it in ArcGIS Online.