Currently you can decide to keep sent items per default, or not or be prompted. It could be interesting to have a fourth option telling to keep a copy of your sent items but without the attachment(s). It/They could be replaced by a placeholder telling which one(s) has/have been deleted such as it is the case when you manually perform the operation.
example : [attachment "xxx.docx" deleted by Mathieu PAPE/xxxxx]
Could save lots of space and time for people that need to clean up their mail boxes.