Remove attachments from sent messages

I've got an older Macbook with a 256 GB HDD drive, running 10.12.6 (Sierra), with Mail 10.3.

It looks like a huge amount of storage (roughly 56 GB worth!) is being taken up by old Mail attachments — mostly attachments to sent messages, not received messages.

Where are these stored, and what is the easiest way to remove them? I've found a lot of reference articles online about deleting the contents of the Mail Downloads folder, but I don't know if that's what I'm looking for, as these are not attachments I received, but rather ones I sent. I don't want to delete the entire message -- I really want to keep a record of what I sent, to whom, and when.

And if possible I'd like to keep the attachments on the server (it's a Gmail account) so that I can retrieve them if need be. What are my options?

This will work only on IMAP accounts. Not POP3

• Quit Mail.

  /Users/<>/Library/Mail/V6 
  

• Navigate there in Finder after turning on hidden files using command+shift+..

• Then search that folder for kind:PDF or kind:Image or anything that you think is taking up space.

• Delete all those search results. Verify that the folder is taking less space, or alternatively check "About This Mac.app" → Storage → Manage → Trash to see that you've actually saved some space. Empty trash.

• Open Mail Preferences → Accounts → Select your account. Under Account Information, set download attachments to None.