ACTION-725: Send a list of content-types for which content transformation applies

Here is a list of content types that a CT proxy may want/need to
transform:

Text and markup:
text/html
application/xhtml+xml  (if not identified as mobile content in DOCTYPE)
application/xml  (if XTHML and not identified as mobile content in
DOCTYPE)
text/xml  (if XTHML and not identified as mobile content in DOCTYPE)
text/plain
application/rtf

De facto standards and proprietary types:
application/pdf
application/msword
application/msexcel
application/vnd.ms-excel
application/vnd.ms-powerpoint
application/mspowerpoint
application/x-shockwave-flash

CSS:
text/css

Images:
image/gif
image/jpeg
image/png

JavaScript:
application/x-javascript
text/javascript


I think this includes the most common content types that would be
transformed, but it's possible I've left some off.  Of course there are
a large number of uncommon content types that a particular CT proxy may
want to transform.

I've left multimedia types (audio, video) off the list for now.  Not
sure if we want to say anything about those types.

Sean