Added support for default frame conversion (image) so that it is now possible to view any page in a PDF file as an image through the browser. Added better support for links in PDF file. Make sure hasBaseData is implemented at File class level.

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@17511 20353a03-c40f-0410-a6d1-a30d3c3de9de