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I believe some of these types are not "official", in the 
sense that they are supported by an RFC; types that are 
not defined in an RFC MUST use the x- prefix. E.g. in what 
RFC is "text/xsl" or "text/xul" defined?

IOW, only types listed in 
http://www.isi.edu/in-notes/iana/assignments/media-types/
should be supported without x-prefix.

Could you please review the entire list of known types 
with this respect, and update the patch accordingly?