Uploads break XHTML

I was playing with the XHTML output customisation which seemed to be
working fine.  However, once i uploaded a file, using the Attach: syntax,
it didn't validate as XHTML.  In fact, my browser (Mozilla Firebird 0.6)
refused to display anything but an error message.  I've taken the
customisation down for the moment, but would like to be able to generate
valid code if possible.

Looking at your test xhtml wiki, I guess you must already be aware of this
problem, as this page
http://www.pmichaud.com/work/xhtml/pmwiki.php/Main/WikiSandbox does not
validate.

Edward Andrews