Add an attachment
At the bottom, there are several tools to use next to the Send button
This one attaches files from your hard drive or network drive. This one attaches files from your Google Drive space.
Once you choose where to get the file, you will be given a box to locate it. Double-click on the name of the file to add it to your message.
Files from your computer will show up at the bottom of the Compose box. Files from Google Drive will show up in the body of the message.
Save an attachment
Attachments will show up at the bottom of a message that you receive. You will see two options: a) to download to your computer or b) to move into your Google Drive space. (These options will show up as you hover over an individual attachment or to the top right, as shown, when there are multiple attachments.)