If you encounter problems with QuickLook not creating Thumbnails or Previews properly, one of the first steps is to discover which code is responsible for generating those for QuickLook. Prior to macOS Sequoia, the standard way to do that was using the command tool qlmanage, among whose options is -m, to list all the qlgenerators available on your Mac. If you’ve tried that in Sequoia, you’ll surely have noticed that no longer works.

qlmanage

Since Catalina, Apple has been encouraging developers to switch away from qlgenerators to app extensions to create custom Thumbnails and Previews for QuickLook, and Sequoia is the first version of macOS that can’t use third-party qlgenerators. I have noticed some document types that only a few weeks ago in Sonoma still used custom thumbnails and full previews, but now can’t do so, although others continue to work normally.

These are controlled in the Quick Look item in Login Items & Extensions in General settings.

That should list all third-party app extensions providing this service, and enabling the right one(s) could fix some of those problems. But it turns out this list isn’t complete, and doesn’t in any case tell you which app extension handles which file type. For those, you’d normally turn to qlmanage, but its -m option can only see the qlgenerators in macOS, and no third-party app extensions at all. In fact, qlmanage is now of little help for anything related to QuickLook. I’ve gone back through Sonoma and Ventura, and qlmanage there is no different: although it does list third-party qlgenerators, none of those provided in app extensions appear in its list.

QuickLook app extensions

As far as I can discover, Apple doesn’t provide any equivalent of qlmanage that can report on QuickLook app extensions. The closest it comes is in the pluginkit tool, that can list all app extensions known to macOS. With a bit of tweaking, its -m option can reveal which of those use the QuickLook SDKs for Thumbnails or Previews.

Armed with the appex bundle path from pluginkit, you can then inspect the Info.plist in each, where there’s an array of QLSupportedContentTypes giving the UTIs of all file types supported by that appex. Although I’m sure someone could implement that in a shell script, this seemed an ideal task for my free utility Mints.

Mints and QuickLook

Version 1.20 of my free utility Mints is now available from here: mints120
from Downloads above, from its Product Page, and via its auto-update mechanism.

This adds a twenty-fifth button to the app’s control window, named QuickLook, at the bottom left. Click on that and Mints will open a new window and fill it with information about all the qlgenerators and QuickLook appexes your Mac knows about.

For qlgenerators, you’re given the file UTI, the path to the qlgenerator file, and (when available) its version number, e.g.
com.adobe.pdf /System/Library/QuickLook/PDF.qlgenerator (1002.2.3)

App extensions are divided into two, the first are those providing Previews, and the second those for Thumbnails, e.g.
com.apple.applescript.text /Applications/PreviewCode.app/Contents/PlugIns/Code Previewer.appex

This is an appex provided in one of Black Pyramid Software’s superb Preview series, in PreviewBundle 2 from the App Store (highly recommended).

You will see a few entries like Safari’s
[none] /System/Volumes/Preboot/Cryptexes/App/System/Applications/Safari.app/Contents/PlugIns/SafariQuickLookPreview.appex
with an appex that doesn’t have a list of file types in QLSupportedContentTypes.

Checking UTIs

It’s easy to guess which UTIs represent many file types, but some are a bit more cryptic. For those, copy and paste the UTI into the UTI field of my free UTIutility and it will give you clues as to its identity, including file extensions.

Unfortunately, some of the system qlgenerators support generic UTIs such as
public.audio /System/Library/QuickLook/Audio.qlgenerator (1002.2.3)
public.image /System/Library/QuickLook/Image.qlgenerator (1002.2.3)
public.movie /System/Library/QuickLook/Movie.qlgenerator (1002.2.3)
which clearly cover broad ranges of more specific file types, but don’t provide any more specific information.

How to identify QuickLook extensions

• List installed QuickLook extensions using Mints’ QuickLook button.
• Identify the file’s UTI using UTIutility.
• Locate the UTI in the list of extensions.
• If no match is found, check UTIs listed in UTIutility as Conforms.
• Check Quick Look item in Login Items & Extensions in General settings, to ensure that extension is enabled.

Next up for Mints is a feature to explore app extensions. I may be a little longer on that one.

QuickLook is the subsystem in macOS responsible for providing two types of document preview, small Thumbnails and full Previews. If you’ve already upgraded to Sequoia, you’ll have noticed that some document types are no longer displayed with their custom Thumbnails or Previews. This article explains what has happened, and how it should work in the future.

As I’ll detail on Saturday morning, QuickLook (or Quick Look) is the latest in a series of methods for providing custom icons and previews for documents, that started back in the initial versions of Classic Mac OS. macOS ships with its own code to generate Thumbnails and Previews for a wide range of standard file types, from text and PDF to audio and movies. To extend these to other types, developers are encouraged to provide their own code.

Prior to macoS 10.15 Catalina in 2019, the display of Thumbnails was supported by the QuickLook framework. From Catalina onwards, this is provided by a new framework named QuickLook Thumbnailing. The older framework is documented here, and had been deprecated for some years. Its replacement is documented here. To extend these, the older framework used QuickLook generators with the extension .qlgenerator, but in the newer framework this function is provided by QuickLook preview extensions, in particular Thumbnail Extensions, that were explained to developers at WWDC in 2019.

As with most deprecated features, eventually the time comes for Apple to remove support for the old, and for QuickLook generators that has occurred in macOS 15.0 Sequoia. From now on, QuickLook Generator plugins no longer work. Oddly, those provided by macOS in /System/Library/QuickLook are still named with the old extension of .qlgenerator, but all custom support now has to use the new framework in App Extensions.

To check whether an app is still trying to use an old QuickLook Generator, look inside the app bundle in Contents/Library/QuickLook. If you see one or more .qlgenerator bundles there, then those no longer work in Sequoia. Instead, you should see new Thumbnail Extensions in Contents/PlugIns, where you should see App Extension bundles with names ending in something like Thumbnail.appex and QuickLook.appex. Some of the better apps provide both QuickLook Generators for compatibility with Mojave and earlier, and App Extensions for more recent macOS.

If the app you rely on to generate custom QuickLook Thumbnails and Previews doesn’t yet come with those App Extensions, contact their Support and ask them when they’re going to implement the changes brought five years ago in Catalina. Particularly if you’re paying them a subscription, it’s time they caught up. Until they do, I’m afraid those Thumbnails and Previews simply won’t work in Sequoia, and you’ll continue to see generic icons rather than Thumbnails.