It looks like that would likely work, but I was hoping there was something written into the WAB already I guess. I'm looking to keep things pretty simple here. This looks more complicated than I was expecting. The thing is, the popup window will show a preview for a hyperlinked image, just not an attachment I guess. Unless I'm misunderstanding something (likely)?
Is that for real?
The WAB doesn't have any functionality for display attachments through a popup?! That just seems absurd?!
Granted, my feelings are weighted by just completing a process where I attached over 2k images to a couple feature classes and now I am looking for how to display them...
The jQuery has the steps you need to go through, but you need to have Web App Builder Developer Edition. I agree with Robert, it could be a lot of trouble, as there is a lot of code in the Developer Edition, but if you have to do it, it is possible.