In all implementations for EMailExtension, the MIME type list is a
static string array -- with the single exception of the text-highlight
module, where the MIME type list is dynamically assembled once.

Replace the mime_types() method with a "mime_types" string array in the
EMailExtensionInterface struct.  Then the list of MIME types supported
by the class implementing the EMailExtensionInterface can be obtained
without requiring an instance of the class.