Symptoms
I appified a list and clicked on the "New Item" button. This opened a new form and I was able to create a new item with an attachment and this all worked perfectly fine. When I edit the "Attachments List" view, edit the "When the View execute Initialize" rule, and click the "Configure" link of the "then execute the Get Attachments List method (Configure)" rule, I noticed that the required "List Item ID" input property was missing. When you cancel that, and OK the rest, it breaks the "New Item" form.
Diagnoses
This was diagnosed as a bug. When you click the "Configure" link of the "then execute the Get Attachments List method (Configure)" rule, the input property is supposed to be "pre-populated", because it is a required field.
Resolution
The latest set of 4.6.9 rollups fixes the issue. After we applied the rollups, the input property was populated as expected and the "New Item" form was working as expected after the edit.