Open Radar

 
The QLThumbnailProvider code in a thumbnail extension on an iOS / iPadOS *16* app fails to create thumbnails, unique to each file, for custom file types.  Those thumbnails are supposed to appear for documents of those types in either the app's own UIDocumentBrowserViewController view or the Files app view.  The exact same code, though, successfully produces visible thumbnails when the app runs on iOS / iPadOS *15*, so the problem seems clearly linked to some bug in iOS / iPadOS 16.

Steps to reproduce:

(1) Compile and run Apple's own "Particles" app sample code, available at https://developer.apple.com/documentation/uikit/view_controllers/building_a_document_browser_app_for_custom_file_formats.
(2) When that code is run on iOS / iPadOS *15.5* devices or simulators, the code successfully produces custom thumbnails for the Particles files that can be created in the app.  You can see those thumbnails in the UIDocumentBrowserViewController view for the app, or in the Files app.
(3) On the other hand, when the exact same code is run on iOS / iPadOS *16* devices or simulators, the code *fails* to produce thumbnails for the Particles files that can be created in the app.  The generic Particles app document icon appears instead for those files in the UIDocumentBrowserViewController view for the app, or in the Files app.