You cannot pass complex objects (i.e. a parsed XML document)
via a form field. You have a couple of solutions:
1) Store your parsed XML document in the user's session scope
2) Pass the unparsed XML text via a hidden form field (you'll
probably want to encode the data to prevent problems with < >
and quotation marks)
3) Only pass the id or location of the XML file, then
retrieve it on the action page
4) Serialize the parsed XML document back to XML text (see 2)
or WDDX