As per this ArcGIS Idea here: https://community.esri.com/ideas/15753-attachment-photo-viewer-widget-in-web-appbuilder
Please add a widget that lets you scroll through attachments for a featureclass. Similar to the details widget in Operations dashboard or the one trick attachment viewer templated app.
I have a feature service for bridges that includes both pictures of the bridges as well as PDF inspection reports. Ideal situation would be where I could select a feature, and a list of the associated attachments would be populated, with display options much like the list widget. Even a list where I could set it to display the fields I add about each attachment would be nice. Ability to click and open an attachment, also a plus. Ability to remove / add even better.