http://www.ietf.org/rfc/rfc3023.txt
This document standardizes five new media types -- text/xml, application/xml, text/xml-external-parsed-entity, application/xml- external-parsed-entity, and application/xml-dtd -- for use in exchanging network entities that are related to the Extensible Markup Language (XML). This document also standardizes a convention (using the suffix '+xml') for naming media types outside of these five types when those media types represent XML MIME (Multipurpose Internet Mail Extensions) entities. XML MIME entities are currently exchanged via the HyperText Transfer Protocol on the World Wide Web, are an integral part of the WebDAV protocol for remote web authoring, and are expected to have utility in many domains.

Major differences from RFC 2376 are (1) the addition of text/xml- external-parsed-entity, application/xml-external-parsed-entity, and application/xml-dtd, (2) the '+xml' suffix convention (which also updates the RFC 2048 registration process), and (3) the discussion of "utf-16le" and "utf-16be".