We don't currently support file attachment on a proposal. However, you can add a link to files on your engagement letter or personalized messages if you have the files uploaded on a website or on a file sharing site like Google Drive or Dropbox.
To add a link, wrap the link text with quotation marks (") and immediately follow it with a colon (:) and the file's URL.
Here's a "link to a file":http://practiceignition.com/file
This will appear in your editor like this: