Saving mail attachments to external drive

I want to save certain incoming mail ***attachments*** to an external drive, not a whole folder or even a whole email. I want to set up a ***rule*** in Mail that will automatically select certain regular attachments I receive and file them in a designated folder on a connected external drive, deleting them from Mail. Clearly, Apple Mail's rules exclude any such capability and only allow filing in another Mail folder. Can anybody suggest a simple way of accomplishing this?