The &is_foo() (e.g. &is_xml()) functions, and the $File->{Type} attribute, make
use of the data (and accompanying logic) from the deprecated conf/type.cfg.

This should be ripped out and replaced with something making use of the new
Types Registry, but it needs to be done carefully since this logic is hairy
and determines XML vs. SGML mode using heuristics and DWIMery.

It's likely the Type Registry needs to be unraveled to provide a Content-Type
 -> Parse Mode (XML vs. SGML) mapping, or to compute it on the fly. Possibly
this wants a third "UNDETERMINED" state for ambigious cases like "text/html"
where we may be parsing XHTML or MathML (which need XML mode and not SGML
mode as the Content-Type alone would indicate).