Can't create a new mime type on Ubuntu 18.04

Question

i want to create a new mime type for idx/sub subtitles. The current mime type is 'text/plain'. I created a file idx.xml in /usr/share/mime/packages with this content

<?xml version="1.0" encoding="UTF-8"?>
<mime-info xmlns="http://www.freedesktop.org/standards/shared-mime-info">
  <mime-type type="video/x-idx">
    <comment xml:lang="en">idx</comment>
    <icon name="bdsup2sub"/>
    <glob pattern="*.idx"/>
  </mime-type>
</mime-info>

and updated the mime database with

sudo updata-mime-database /usr/share/mime

A new xml file is created in /usr/share/mime/video with the name 'x-idx.xml'. The content is

<?xml version="1.0" encoding="utf-8"?>
<mime-type xmlns="http://www.freedesktop.org/standards/shared-mime-info" type="video/x-idx">
  <!--Created automatically by update-mime-database. DO NOT EDIT!-->
  <comment xml:lang="en">idx</comment>
  <icon name="bdsup2sub"/>
  <glob pattern="*.idx"/>
</mime-type>

There is also the new mime type in the Xfce Mime Type editor. But when i check the idx file again the mime type is still

text/plain

Rebooting the computer doesn't change anything. What else can i do? Do i need to do anything with that 'magic number' that can be used to detect a mime type?