I'm using CCK's filefield module and have a multi-value file field on my custom content type. It's actually an imagefield, but I don't think that should make any difference. I can easily migrate one filefield entry but am unsure how to handle it when multiple files need to be migrated for the one field. I'm trying to do it in a hook_migrate_prepare_node() function, but don't know what to set the $node->field_file_sourcefile variable too. I'm guessing it needs to be some sort of an array or object.