Major Description
The javadoc for the method suggest that it will return the "preferred extension" for a MimeType. Acutally its the first Extension deserialized from tika-mimetypes.xml. The method would have to be renamed to getFirstExtensionInList(). This pretty much points at the fact that this method makes no sense at all.
There are several use-cases in which one would want to know ALL known valid extensions for one mime type. I will submit a patch later today.