I have spent some rather lengthy time in trying to get this right, and I finally have.

Creating the mimetype

First, you need to create a mimetype file and save it to your local ~/.local/share/mime/packages/ directory. The file can have a structure like this one:

<?xml version="1.0" encoding="utf-8"?>

<mime-info xmlns="http://www.freedesktop.org/standards/shared-mime-info">

  <mime-type type="text/todo">

      <comment>To-do list</comment>

      <glob case-sensitive="true" pattern="TODO" />

      <glob case-sensitive="true" pattern="TODO *"/>

    </mime-type>

</mime-info>

Be sure that fields like type in the mime-type element tag match your desired mimetype, and the glob element tags contain a pattern attribute whose value matches the pattern of your disired mimetype. As you can see, your mimetype can cover multiple filename patterns through multiple glob elements.

In the case of my TODO mimetype, I needed the pattern to be case-sensitive, so I threw in a case-sensitive="true" attribute-value pair; if you don't need case-sensitiveness, this attribute-value pair is not necessary.

Once you have finished your mimetype file, save it with a convenient name, like todo.xml. (Note: the XML extension might be important.)

Now run the command update-mime-database ~/.local/share/mime/ and you should be done with it. This command will place your mimetype in the correct category directory under ~/.local/share/mime.

Testing

You can test if your mimetype was parsed correctly by reading the ~/.local/share/mime/glob files and by testing specific files with commands like

• mimetype FILENAME


• xdg-mime query filetype FILENAME

These will only work after you've updated the mime database.

Adding the icon

In many linux distributions, third-party/addon icons are filed under the "hicolor" icon-theme, which is a fallback icon theme which other icon themes rely upon.

Add your icon to the correct directory under ~/.local/share/icons/hicolor/ (according to the /usr/share/icons/hicolor/index.theme descriptor file that lists valid subdirectories), run update-icon-caches ~/.local/share/icons/hicolor and you should be all set!

Your desktop manager may require a restart to associate the new mimetypes with the new icons.

Troubleshooting

If your files don't get the mimetype/icon you set for them, be sure to check very well the patterns in the mimetype XML file. Sometimes testing an obvious pattern may help you verify this.