Apple - QuickLook for files with no extension or an unknown extension

Take a look at the QuickLook Stephen plugin, it opens almost everything I throw at it (as long as it is text based).

Update 2021 Jan

If you are running macOS Big Sur (11.1) and you can't get QuickLook previews of certain files even if you installed QLStephen and other plugins, this is what worked for me.

Background

In macOS, each file is assigned a UTI. UTIs are used to identify file types (these are the types you see in the Kind column in Finder or in the Kind entry in the General section of the Get Info window). For example, a .txt file (a "plain text" file) has the following UTI: public.plain-text.

To check which is the UTI of a particular file, you can run:

mdls -name kMDItemContentType ~/my-file.ext

where ~/my-file.ext is the path to the file.

QuickLook checks the UTI of a file to choose a QuickLook generator to use for display. The QL generators that get shipped with macOS can be found in /System/Library/QuickLook/.

QL generators have the .qlgenerator extension and you can see their content by right-clicking on them and selecting Show Package Contents. Inside the folder Contents there a file named Info.plist. This file lists which UTIs that generator will be used with.

So, the Info.plist of the Text.qlgenerator will list, among others, the public.plain-text UTI. Any file that has that UTI will be previewed using Text.qlgenerator.

The generators found in /System/Library/QuickLook/ are locked, meaning that you cannot (in principle) edit them.

This is where the QuickLook plugins that can be found on the internet, like QLStephen, come in. These plugins are installed in ~/Library/QuickLook (note that this path starts with ~/, meaning it's your user Library and not /System/Library/) and extend QuickLook capabilities. In other words, they provide the system with new generators that work with UTIs not covered by the system's own generators, or extend them.

Problem

When you reassign the application a file type is opened with (e.g. Info > Open with > [app] > Change all`), the application might assign that file type a UTI that is not the default, meaning that QuickLook won't recognise the UTI and won't assign any generator (and you just see a file icon rather than the content of the file).

For example, the plugin QLColorCode should preview the content of LaTeX .tex files, but in my case it wasn't. This is because I selected Sublime Text as the app that opens .tex files, and ST has assigned a UTI that is different from the UTI used in QLColorCode for .tex files.

Solution

The solution to the problem is simply to let the QL plugin know that it should also work with these "non-default" UTIs.

To do so, you just need to edit the Info.plist file of the plugin in ~/Library/QuickLook (do not try to edit the system generators in /System/Library/QuickLook).

More specifically, you first have to locate the following lines in Info.plist (to open and edit this file, you right-click on it and select TextEdit):

...
<key>LSItemContentTypes</key>
  <array>
    <string>public.source-code</string>
    ...
  </array>
...

Then, you have to add the "non-standard" UTIs in the array, like so:

...
<key>LSItemContentTypes</key>
  <array>
    <string>public.source-code</string>
    ...
    <string>dyn.ah62d4rv4ge80g5dx</string>
    <string>dyn.ah62d4rv4ge81e5pe</string>
  </array>
...

As mentioned above, to get the UTI of a particular file, do:

mdls -name kMDItemContentType ~/my-file.ext

In the output, you will see something like this:

kMDItemContentType = "dyn.ah62d4rv4ge80g5dx"

The string between double quotes is the UTI of ~./my-file.

After you have added the UTIs, just save the Info.plist file and close it.

Now QuickLook should work with the chosen generator for the files whose UTIs have been added in Info.plist.

To be on the safe side, you can run the following to reset QL and its cache:

qlmanage -r
qlmanage -r cache

Fin