Please check if the below approach works for you:
- Message can be a different entity and associated with FileDocument
- When you want to reset, you can clone the Message without associations
So FileDocument does not gets re-created
Or, you must develop a logic that determines whether the object is duplicated and not edited (like isNew() in mendix)
- if yes, when loading the screen you must delete the document, which in my opinion is a long way for the solution you need