Once you've logged into your Yahoo! Mail account and begin composing a message, follow the instructions below to add an attachment:

1. Click the "Attach Files" link located above the compose window.
2. Click the "Browse" button and select a file.
3. When you have selected a file, click the "Attach Files" button located towards the bottom of the page to attach it to your message.
4. You will then see a file attachment confirmation page that will allow you to remove an attachment, if needed, by clicking on the "Remove" link located to the right of the file.
5. Click on the "Done" button to attach the selected file(s) to your message