To delete an attachment on an Action Item, go to Action Items > All Action Items or My Action Items and find the Action Item where you need to delete the attachment.
Click on the black arrow (highlighted below) of the Action Item to open the messages dropdown and then click on the green paper clip to view the attachment.
A separate Attachments window will appear. If there are more than one attachments, you can switch between different attachments on the action item by selecting the Prev and Next buttons. Click the arrow button on the left of Prev (highlighted).
You will then see the options for Delete and Regenerate.
Select Delete and a pop up message will appear. Select OK to Delete.