Use DirectoryNamer on PropertyMapping::getUriPrefix()

Using DirectoryNamer, we faced an issue :
Upload path was fine, for example "/uploads/485/something_64efz5f.jpg", but as "485/" part of the path is generated by the DirectoryNamer, the resolvedUri result as something like "/uploads/something_64efz5f.jpg".
So unless we are missing the right way to use it, this should fix it.