In order to attach files to the message, click the Attach button in the toolbar and then select the file on your computer using the file picker dialog that opens. Attachments to be sent with the message are listed on the right and can be removed again by clicking the Delete icon of the according file.

If your browser supports HTML5, you’ll see the file-drop icon at the bottom of the attachment list. That means you can drag a file from your computer’s file explorer with the mouse onto that icon in order to attach it to the message.