To send a message with an attachment, you should use the Materials functionality instead of the messages functionality.
Here are more help topics about Materials
With materials, you can upload a document, image, spreadsheet, etc., and request a notification be sent about that documenting.
You can choose whether to include the material as an attachment to the email, or keep it as a clickable link that the when clicked, will download the material to the recipients computer.
If you have already uploaded a material, you can "Edit" it and select to send a notification and include it as an attachment. This allows you to send the material multiple times.
NOTE: There are some size restrictions on what can be included as an attachment.