file field of copied event updates field value of original event

When you copy an existing event with a custom data set including a file field, changing the value of the copied event also changes the file value of the original event. They seem to be connected.

You can reproduce this on the demo website:

1. Create a custom data set called "Attachements" for events
2. Add a field of type file to this set
3. Choose an existing event like "Fall Fundraiser Dinner" and upload a file with this new custom data field
4. Copy this event
5. Edit the copied event and upload a different file
6. Check the original event. Both have been updated

Removing the file in the copied event results the same.

The attachment of the original event seems to be broken after updating the attachment of the copied event: