getConversion return a list of two values, so previous code ("return
len(self.getConversion(**kw))") always return 2.
Now the len of the data is returned.
In case of images, data is an OFSImage object. But OFSImage objects support
len, so it works also in case of images.

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@31542 20353a03-c40f-0410-a6d1-a30d3c3de9de