Thanks Aravisian, I was able to fix do it!
This is a full guide of how I did it:
- I downloaded an .svg icon
- Change the icon's name to the mimetype of the file (right click > properties > type), in my case 'application-x-blender.svg'
- Move the icon to the correct folder:
$ sudo mv '/home/hanne/Downloads/blender file icons/application-x-blender.svg' '/usr/share/icons/Zorin/scalable/mimetypes/'
If the folder does not yet exist, you may create it. - Update icon caches:
$ sudo update-icon-caches /usr/share/icons/Zorin/
If you don't use the default theme, replace the 'Zorin' in the command with the folder of your theme.