Add a custom MIME type for all users

Create the /usr/share/mime/packages/application-x-newtype.xml file:

<?xml version="1.0" encoding="UTF-8"?>
<mime-info xmlns="http://www.freedesktop.org/standards/shared-mime-info">
  <mime-type type="application/x-newtype">
    <comment>new mime type</comment>
    <glob pattern="*.xyz"/>
  </mime-type>
</mime-info>

The sample application-x-newtype.xml file above defines a new MIME type application/x-newtype and assigns file names with the .xyz extension to that MIME type.

Create a new .desktop file named, for example, myapplication1.desktop, and place it in the /usr/share/applications/ directory:

[Desktop Entry]
Type=Application
MimeType=application/x-newtype
Name=My Application 1
Exec=myapplication1

The sample myapplication1.desktop file above associates the application/x-newtype MIME type with an application named My Application 1, which is run by a command myapplication1.

As root, update the MIME database for your changes to take effect:

# update-mime-database /usr/share/mime
        

As root, update the application database:

# update-desktop-database /usr/share/applications
        

To verify that you have successfully associated *.xyz files with the application/x-newtype MIME type, first create an empty file, for example test.xyz:

$ touch test.xyz

Then run the gio info command:

$ gio info test.xyz | grep "standard::content-type"
  standard::content-type: application/x-newtype

To verify that myapplication1.desktop has been correctly set as the default registered application for the application/x-newtype MIME type, run the gio mime command:

$ gio mime application/x-newtype
Default application for “application/x-newtype”: myapplication1.desktop
Registered applications:
	myapplication1.desktop
Recommended applications:
	myapplication1.desktop