If your system sees "application/xml" and has been configured to hand it
off to a program "XML" (you can certainly configure Mozilla in that way)
does the "XML" program get to see the complete message, including the
Content-Type header (or does it just get to see the body).
Few MIME handlers are capable of this. It is something that all of them should
support, but there's a gap between what should be available and what is.
Now, some handlers have limited capabilities of this sort. For example,
mailcap-based handlers often have the ability to put a particular parameter's
value on the command line using a substitution scheme.
Because it "XML" gets to see the full Content-Type header, then you just
make XML a shell script that examines the Content-Type header, spots the
foobar and calls the foobar handler or, alternatively, if it doesn't see
the foobar (or doesn't have a foobar handler to hand) then it calls the
REAL (generic) xml handler.
Sorry, you cannot count on this ability.