Hints for mkvtoolnix-gui in main

Warnings

• internal-unknown-tag
  The generator emitted a tag 'internal-unknown-tag' which is unknown. This is a bug in the metadata generator, please file a bugreport.
• asv-mimetypes-tag-deprecated
  :33 -
  The toplevel `mimetypes` tag is deprecated. Please use `mediatype` tags in a `provides` block instead to indicate that your software provides a media handler for the given types.