To attach a file:

Choose Message > Edit Attachments...

Click the Add (+) button at the bottom left of the window.

Select the file(s) you want to attach and click "Attach".

Click OK.
You can attach any kind of file to your messages. In addition, you can drag and drop images, sounds, and movies into the body of your message.
Choose Message > Edit Attachments...
Click the Add (+) button at the bottom left of the window.
Select the file(s) you want to attach and click "Attach".
Click OK.