On Ubuntu 10.10, which XML file under the directory /usr/share/mime/applications is for MIME type text/html?
I checked xml.xml and xhtml+xml.xml, but neither has a glob pattern of *.html.
So where is this file format entry? 
This should help. This search was done on Debian 6, but should be the same on Ubuntu.
#$find /usr/share/mime/ -print0 | xargs -0 grep [.]html 
/usr/share/mime/globs2:50:text/html:*.html
/usr/share/mime/packages/freedesktop.org.xml:    <glob pattern="*.html"/>
/usr/share/mime/globs:text/html:*.html
/usr/share/mime/text/html.xml:  <glob pattern="*.html"/>
From this you can see the information is in /usr/share/mime/globs, /usr/share/mime/globs2, /usr/share/mime/text/html.xml and /usr/share/mime/packages/freedesktop.org.xml