If this mimetype is a subclass of one or more other mimetypes, return the list of those mimetypes. For instance a application/javascript is a special kind of text/plain, so the definition of application/javascript says sub-class-of type="text/plain" Another example: application/x-shellscript is a subclass of two other mimetypes, application/x-executable and text/plain. (Note that this notion doesn't map to the servicetype inheritance mechanism, since an application that handles the specific type doesn't necessarily handle the base type. The opposite is true though.)
