How to associate file extension (not file type) for a particular application?

Rename the file you have as _anything_.task (where _anything_ is something suitably descriptive.)

First you need to create an XML file with the extension information in it. (If you wish, copy the text below into a new text file and save it as task.xml.)

<?xml version="1.0" encoding="UTF-8"?>
<mime-info xmlns='http://www.freedesktop.org/standards/shared-mime-info'>
<mime-type type="application/taskcouch">
<comment>TaskCoach File</comment>
<comment xml:lang="bg">Taskcoach</comment>
    <comment xml:lang="cs">Taskcoach</comment>
    <comment xml:lang="de">Taskcoach</comment>
    <comment xml:lang="es">Taskcoach</comment>
    <comment xml:lang="eu">Taskcoach</comment>
    <comment xml:lang="hu">Taskcoach</comment>
    <comment xml:lang="it">Taskcoach</comment>
    <comment xml:lang="ko">Taskcoach</comment>
    <comment xml:lang="nb">Taskcoach</comment>
    <comment xml:lang="nl">Taskcoach</comment>
    <comment xml:lang="nn">Taskcoach</comment>
    <comment xml:lang="sv">Taskcoach</comment>
    <comment xml:lang="uk">Taskcoach</comment>
    <comment xml:lang="vi">Taskcoach</comment>
<glob pattern="*.task"/>
<alias type="application/taskcoach"/>
</mime-type>
</mime-info>

You now need to save or copy this new file into the directory

• ~/.local/share/mime/packages for a per-user file association or
• /usr/share/mime/packages for a system-wide file association.

Once that is done, run

update-mime-database [MIME-DIRECTORY]

where [MIME-DIRECTORY] is the previously chosen directory minus the /packages suffix. (Use sudo for the system-wide association.)

Now, open your file manager and right-click on anything.task and select taskcoach as the default program with the open with option in the context menu.

Job done. All .task files will now open with taskcoach!

The file opens in TaskCoach, but any other .XML file gets opened with TaskCoach too!

That happens because they’re all the same MIME type (application/xml).

If you want TaskCoach files to be treated differently from other XML files, you need to configure a new MIME type for them.

I’ve never done that before, but it looks like Gnome has pretty good documentation on how to do it: Add a custom MIME type for all users / individual users.