The Attachment Viewer app requires a feature layer with attachments. It includes the capability to view attachments of a hosted feature service or an ArcGIS Server feature service (10.8 or later). Currently, the app can display JPEG, JPG, PNG, GIF, MP4, QuickTime (.mov), and PDF files in the viewer window. All other attachment types are displayed as a link.
https://doc.arcgis.com/en/instant-apps/latest/create-apps/attachment-viewer.htm
