Artwork description is not being saved to file

The API allows adding a description to album artworks. However, if I read the tag afterwards with an external program such as MP3Tag, it doesn't show the artwork description. Seems like it's not being written properly to the audio file. This is the code I am using:

Artwork artwork = ArtworkFactory.getNew(); artwork.setDescription("my description should be displayed here"); ... mp3File.save();