It seems ArcGIS Server (using 10.7) is assigning MIME type application/octet-stream when clicking pop-up links for photo attachments on hosted feature layers.
Instead of opening a new window/tab and previewing the photo, the browser is opening the download file dialog box.
This issue only seems to happen on attachments for hosted feature layers. Attachments for feature services present the files as image/jpeg and load into the new tab properly.
I have not tested this out with any other file types but I assume it will handle them the same way.
It would be nice if Server could feed proper MIME types for hosted layers too. Previewing images in the browser window is incredibly important to our workflow. Also, hosted layers represent the best way to create and organize content for our organization. Copying data to the server through feature services just really isn't an option.