How do I send a message with an attachment?

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 documents, and request a notification be sent about that documenting, deciding whether to include the document as an attachment (there are size limitations) or as a link.

You can also upload the material, then copy a link to the material and send that in a message.