The problem

I frequently work with 3D models in the web and therefore with .gltf or .glb files. These formats are the best choice for compressing and optimizing models for the web but they still seem to lack support in operating systems and their file managers. That is also the case for the preview icons. For example, that's how a .gltf file shows up in the default file manager in Fedora 38, which is Nautilus:

That's obviously not very helpful. It's blurry, shows a warning symbol and does not really represent the file's contents.

I was wondering if it's possible to change icons for specific file types (also called MIME types) and as it turns out, it's possible!

What to do

First, it's important to make the file extension known to the operating system. You can find existing custom definitions for all users in the /usr/share/mime/packages folder:

Notice the gltf.xml and glb.xml files? For every file type you want to add a custom icon to you need to add a corresponding .xml file. Let's look at the glb.xml file for reference:

<?xml version="1.0" encoding="UTF-8"?>
<mime-info xmlns='http://www.freedesktop.org/standards/shared-mime-info'>
	<mime-type type="model/gltf-binary">
                <comment>glTF Binary Format MIME Type</comment>
                <glob pattern="*.glb"/>
        </mime-type>
</mime-info>

The important part is the <mime-type type="model/gltf-binary"> and the <glob pattern="*.glb"> line. Add a new file and change the lines according to your MIME type and file extension and also name the file accordingly.

Update the mime database

The next step is to update the mime database to give your defined MIME types to files ending with .glb or .gltf.
You can do that with:

sudo update-mime-database /usr/share/mime

This will generate a folder with your MIME type xml definition in /usr/share/mime/, so for .glb and .gltf it will create the following files:

luca@carbon /usr/share/mime $ tree model
model
├── 3mf.xml
├── gltf-binary.xml
├── gltf+json.xml
└── ...

1 directory, 4 files

Add the icons to your file types

The final step is getting your .svg icon ready and connect it to your MIME type. Copy your svg icon to /usr/share/icons/hicolor/scalable/mimetypes/<mimetype-group>-<mimetype>.svg. For .glb the mimetype-group would be model and the mimetype would be gltf-binary, resulting in this line for the whole filepath:

/usr/share/icons/hicolor/scalable/mimetypes/model-binary-gltf.svg

Now you just need to update your icon database with sudo update-icon-caches /usr/share/icons/* on Debian/Ubuntu or sudo gtk-update-icon-cache /usr/share/icons/* on Fedora. You may need to restart you system to get the icon to show up.