One of the novel features in the original Finder in Classic Mac OS was the use of distinctive icons for different types of document in an extensible scheme.

Every file had its type and creator codes, each consisting of four single-byte characters. The Desktop databases contained indexes to those, to enable the Finder to display the appropriate icon for a text document of type TEXT created by an app with the creator code of ttxt, SimpleText, for instance.

Apps provided a custom icon in their Resource fork for each type of document they supported. Periodically, those Desktop databases became broken, and documents lost their custom icons. The solution was to rebuild those Desktop databases from the data in each app’s Resources, a procedure that every Mac user became only too familiar with.

At some stage, perhaps in System 6 of 1988, or System 7 of 1991, document icons such as images could be displayed as miniatures or thumbnails instead. This was accomplished by apps creating that file’s thumbnail and saving it as an ICN# resource in the file’s Resource fork. Amazingly, this still works in Sequoia, where I pasted a prepared Resource fork into a Zip file to give it an inappropriate thumbnail.

The raw Resource fork is shown below in xattred as a com.apple.ResourceFork extended attribute.

Initially, Mac OS X continued a similar system, including custom thumbnails, until Apple introduced Quick Look in Mac OS X 10.5 Leopard, in 2007. This came with built-in support for a wide range of common document types, extending to QuickTime media including audio and video. One curious omission at first was that animated GIFs weren’t supported as animations until OS X 10.7.

Display of Thumbnails used the QuickLook framework documented here. This enabled third-parties to extend coverage to their own document types using QuickLook generators with the extension .qlgenerator. Initially, they were installed into /Library/QuickLook from each app bundle.

Normally, when QuickLook generated a Thumbnail or Preview, that was stored in its cache database kept in NSTemporaryDirectory in the path C/com.apple.QuickLook.thumbnailcache/. Those could give revealing insights into images and other documents accessed recently, and Wojciech Regula and Patrick Wardle discovered that, in High Sierra and earlier, it was easy for malicious software to examine that cache. Apple addressed that in macOS 10.14 Mojave by making the cache completely inaccessible.

In-memory caching of Thumbnails has also proved controversial in more recent versions of macOS. To deliver smooth scrolling of Thumbnails in the Finder’s Gallery views in particular, the Finder has taken to caching them in memory for up to two days, sometimes using several GB in the process. That can readily be mistaken for a memory leak, until those cached Thumbnails are finally flushed.

I described how QuickLook Thumbnails worked in early 2019, in the days before the SSV.

When you select a document in the Finder, a dialog, or somewhere else where you expect its icon to be shown, the Finder passes details of the document path and its type (UTI) to IconServices, to fetch the appropriate icon. This calls on its main service, iconservicesd in /System/Library/CoreServices, to check its icon cache.

Although the main icon store is locked away in /Library/Caches/com.apple.iconservices.store, there’s additional data in a folder on a path based on /private/var/folders/…/C/com.apple.iconservices, where … is an unreadable alphanumeric name. For icons used in the Dock, their cache is at /private/var/folders/…/C/com.apple.dock.iconcache. If the icon should be replaced by a QuickLook Thumbnail, such as in a Finder column view, QuickLook is asked to provide that thumbnail. That in turn may be cached in its protected cache at /private/var/folders/…/C/com.apple.QuickLook.thumbnailcache.

QuickLook then relies on there being an appropriate qlgenerator to create a thumbnail of that document type; if the qlgenerator is flawed or can’t cope with the document’s contents, that could easily fall over. For example, if you renamed a text file with a .jpeg extension so that macOS considered it was a JPEG image, the bundled qlgenerator might have simply resulted in the display of a busy spinner, rather than resolving to a generic JPEG document icon. IconServices should then deliver the appropriate icon back to the Finder to display it.

In macOS 10.15 Catalina (2019), Apple started replacing this system with a new framework named QuickLook Thumbnailing, documented here. That replaces qlgenerators with QuickLook preview extensions, in particular Thumbnail Extensions, as explained to developers at WWDC in 2019.

macOS 15.0 Sequoia has finally removed support for qlgenerators. That has resulted in the unfortunate loss of custom Thumbnails and Previews for document types of third-party apps that are still reliant on qlgenerators, and haven’t yet got round to providing equivalent app extensions. It’s almost as if the Desktop databases need to be rebuilt again.