Drag the file from Finder into the body of the message in Mail; it attaches where you drop it.
Drag it in
- Start a new message in Mail.
- Drag the file from Finder into the message. If Finder's window is in the way, start the drag, press ⌘Tab to switch to Mail, and drop it there.
Use the attach button
- In the new message, click the paperclip button in the toolbar and choose the file.
Copy and paste
- Select the file in Finder, press ⌘C, then click in the message and press ⌘V.
With dragand
Drop the file on dragand's Hold zone while you write. When the message is ready, press ⌃⌥⌘S and drag the file in. The copy clears itself after 24 hours, so nothing is left to tidy.