To attach files to outgoing email, first find and click the "Browse.." button beneath the text entry box.
Next, find and select the desired file on your computer and then click "Open" in the Choose file dialog:
Once the file is in the "Attach:" box to the left of the "Browse" button, click the "Add" button:
The file should now show up beneath the box where you added it. Larger files might take a little time to load. You may add multiple files up to 10 MB. Note that some email accounts will not receive more than 5 MB files.