How to restrict adding/modifying/ removing attachments based on status
Accepted answer
Yes, this requires a work item save advisor that you have to implement that check on work item save if there are changes to the attachments and if this should be allowed. https://rsjazz.wordpress.com/2014/05/26/only-owner-can-close-workitem-advisor/ is a simple advisor that could be used as starting point. Please note, you will have to compare the attachments in the old state and the new state to find out if there are changes and what they are.