In all major email clients (Gmail, Thunderbird, Outlook, Mailbird, and Apple mail with the most popular extension Mailbutler), while writing a message, if you have mentioned an attachment and click send without attaching anything, there is a confirmation dialogue that reminds you to attach a file. Something like that would be neat in Mail also.

Here is an example of how it looks in Gmail

cc @jancborchardt who also did the gap analysis and @karlitschek