OK - to clarify, you want the 'auto reply' to have this extra file, sent back to the user rather than just the user-submitted files as attachments (that's not what I understood from your original question).
I don't believe you can add the attachment you want, especially not the way you specify. William's way - adding a link to the auto-response email template seems to be the only logical and best way to do so.
I expect it could be done but I don't believe Machform offers free modifications like that where modifying the core code is required.
You could put it on the "wish list" as a feature, to have a custom file to be added to the auto-reply in the configuration screens.