There is a discussion about a few techniques on this GeoNet post:
I think there is going to be better support for this in our new Map Viewer. It is getting a major over haul and the beta will be released any day now. You'll be able to try out the beta without having to sign up or anything. You can read about it here.
I read this at the bottom of the post: Show attachments in pop-ups to streamline the viewing experience and eliminate the need to use Arcade or click a link to view the attachment.