Symptoms
We used the "File" field part to get an attachment from a smartobject and that is mailed to users in a client event notification mail. This solution works perfectly in the source environment, but as soon as we deploy this to the target environment, this breaks when we run through the wizard in the designer again.
Diagnoses
After some investigations, we noticed that the smartobject GUID was not being "refactored" to use the smartobject GUID on the target environment. (It was still using the GUID of the source smartobject.)
Resolution
This ticket was sent to LABS and a fix was provided. If you run into this issue, please log a new support ticket and reference this article in there.