Media type typically expressed using a MIME format (see IANA site and MDN reference) e.g. application/zip for a SoftwareApplication binary, audio/mpeg for .mp3 etc.).
In cases where a CreativeWork has several media type representations, encoding can be used to indicate each MediaObject alongside particular encodingFormat information.
Unregistered or niche encoding and file formats can be indicated instead via the most appropriate URL, e.g. defining Web page or a Wikipedia/Wikidata entry.