I would like to confirm whether you just want to delete the attachments which detached from the email.
In the Exchange there is no such feature could reach this goal about only delete the attachments but retain the email. Additionally, the policy doesn’t touch the attachment but the email based on the time.
In outlook, we can apply with the AutoArchive feature to delete the email which exceeds the limited time or create new managed content settings in Managed Default Folders.
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