Steps to reproduce

1. Make sure the anonymous has no permission to read form submissions
2. Create a form with an upload widget
3. Log out from the backend
4. Submit the form with a file attached
5. See a blank page
6. See the attachment created in the backend

I tracked down the cause to the event handler, that invokes the PublishContent Event which is handled by the `StartWorkflowSubscriber`. There it tries to read the content, but does not have permission to (which is wanted).

Suggested Solutions:

a) Use sudo in the EventListener to always be able to read the content
b) try-catch failed events, log them and continue.