Create an automation rule that will remove the attachments of all issues that were resolved 30 days ago.
Create a new rule and name it appropriately.
Providing a description will help you to identify what the rule does but this step is optional.
Select Scheduled trigger
Schedule the trigger once a day.
Run the automation at 3:00 am as it is preferable to make this operation outside of business hours.
Select JQL selector
Use the following expression:
status WAS closed BEFORE -30d
Select Boolean condition
%{selector.issue.attachment} != null
Select Update field
Select Attachments (current attachments will be replaced)
Select Value returned by parser expression and fill the field below with an empty string (empty quotations marks "").
Update field action