The toolbar buttons are used to send a message and for many other functions needed often and quickly. Of course, there exists a toolbar button for attaching a file to an email message, too.

Start with your message without the attachment. Move the cursor over the buttons on the toolbar and read the tool-tips. Stop when you reach the enormous clip.

Upon clicking that button with the clip, a dialog entitled "Insert Attachment" is opened. Select file and then click "Attach" to email the file.