Change ALLOW_ATTACHMENTS from 0 to 1 to turn on attachment support. You'll note on the send a message page that there's an additional tab labeled Attach. Click on this tab while preparing your message to add attachments. The attachment tab will display the current (server-set) size limits and allow you to browse for new files to attach. Attachments on text format messages are accessed via a URL included at the bottom of the message. Since phpList 3.4.2 those links will open attachements directly from the browser.
