Up to now, when creating a new email message in Gmail, you’ve been able to include documents stored in your Google Drive account by clicking on the Insert files using Drive button.
The trouble is, of course, that if you subsequently remove that file from Google Drive, you also remove access to it for the recipients of your email.
Google is now enabling the option for you to instead share files you have in Google Drive as attachments. When you click on Insert files using Drive you’ll see the button in the lower right corner of the insert window.
Clicking Attachment adds your selected file as an attachment, instead of a Google Drive link (though inserting files as Drive links will continue to be the default).