Mac storage basics · Mail, Messages and chat apps
Where does Mail save attachments I open, and can I delete them?
Short answer: In a Mail Downloads folder inside Mail's container in your Library. The copies are only for viewing, so you can clear them with Mail quit; attachments stay in the emails.
When you open an attachment, Mail saves a copy in a Mail Downloads folder inside its container in your Library. These copies are only for viewing; the original stays in the email.
You can delete the contents of Mail Downloads when Mail is quit — you'll still be able to open the attachments from the messages. Storage Studio lists Mail Downloads among the system leftovers in What's Using Space.