The mime type for a graphic, multimedia object, or piece of supplementary material
Authoring Note: This attribute should be required when creating new books.
The source for this attribute’s “image” value is the Internet Assigned Numbers Authority (IANA 2003).
Although this Tag Set cannot enforce it, the element <graphic> should be limited to still images. The element <media> should be reserved for objects whose @mime-subtype involves video, audio, moving images, time-dependent graphic types, and other non-XML data types. Mimetypes and subtypes should be set accordingly but are not constrained by this Tag Set.