are matching transform policy

As an example: pdf to text conversion force a transformation
to intermediate HTML format, because w3m_dump is a requirement to output plain/text.
This patch allow pdf to text conversion to use pdf_to_text transform,
and let html to text conversion using w3m_dump, honouring configured policy.


git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@39346 20353a03-c40f-0410-a6d1-a30d3c3de9de