* Use accessor to get content type (default value is 'application/unknown').
* Revert r23844 because it would fail later anyway.
* Access data member directly instead of using getData because OFS images doesn't have getData.

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@23969 20353a03-c40f-0410-a6d1-a30d3c3de9de