Use prefetch related to trigger polymorphic downcast.

Under some circumstances, the filer File object is not correctly casted to the relevant child class.
Using prefetch_related triggers the casting more reliably.
This may cause a little performance hit, but the whole `file` attribute (the only 'real' case in cmsplugin-filer for this code to be executed) is going to be retrieved anyway in nearly all cases so this is not going to be much relevant