Events with file custom fields don't copy the files
This same issue was reported in https://issues.civicrm.org/jira/browse/CRM-20029 and I guess it was fixed at some point, but it's back.
Description copied from there:
When an event with a custom file field is copied the new event has a new file id but the id points to the same file on the server as the original event pointed to. If either event is edited and the file is deleted, the other file link becomes broken. The attached file should be copied on the server when the event is copied so that each event has its own file.