[Qt-creator] adding a new mime type

[Christoph Mathys]

I try to add a new mime-type for protobuf/grpc so I can specify it for
the Beautifier-Plugin. But I cannot get it to work. What I tried
(Debian-System):

1. Create /usr/share/mime/packages/protobuf.xml containing:
<?xml version="1.0" encoding="utf-8"?>
<mime-info xmlns="http://www.freedesktop.org/standards/shared-mime-info">
  <mime-type type="text/x-protobuf">
     <comment>Protobuf IDL</comment>
     <sub-class-of type="text/plain"/>
     <glob pattern="*.proto"/>
  </mime-type>
</mime-info>
2. Execute "sudo update-mime-database /usr/share/mime

However, the file extensions does not show up in
Tools->Options->Environment->MIME Types.

Any ideas what else to try?

Thanks!
Christoph