We are expecting more specific MIME types emerge for particular types 
of XML documents.
In section 3 of the draft, we have a paragraph as below:
   XML provides a general framework for defining sequences of
   structured data.  In some cases, it may be desirable to define new
   media types which use XML but define a specific application of XML,
   perhaps due to domain-specific security considerations or runtime
   information. This document does not prohibit future media types
   dedicated to such XML applications. However, developers of such
   media types are recommended to use this document as a basis.  In
   particular, the charset parameter should be used in the same manner.
Makoto
 
Fuji Xerox Information Systems
 
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E-mail: murata@apsdc.ksp.fujixerox.co.jp