event custom data file field not copied

1. create a custom data set for events
2. create a field of type file
3. create an event and select a file in this field
4. go to the manage event screen and copy the event
5. view the event info screen and visit the manage event form

the file is not copied, but a reference to the original entity_file id is retained. because that reference is for the original event/image it doesn't match the new event's ID and won't work.