Good trick, Thanks to you and to Coxorange!
But the trick just removes attachments from the email and not from the Mail folder, alas
user Library/Mail/V4 folder:
Since -at least-the last Sierra update 10.12.6 they appear along with the email they belong to, before they didn't.
Today I did the trick and I tried to look for the image I just deleted from the email, if it stil was in the Mail folder
and there it was!
Now, before deleting the image from the email, I copy its name and then look for it in the Mail folder and delete that one as well.
P.s. About your "cleaning " method
I figured out an easier way to do it, just creating a folder On My Mac, called Remove Attachments
I move one sent email (to myself) with an attachment and the others received that need the cleaning.
Then I select them all and the removal it's easy as you suggested.
Then I put back the emails where they belong.