Hey @admin29! There isn't a built-in method of adding file downloads to buttons at the moment (but feel free to file this as a feature request for our team here40!), but you could still add the file to the email as a link (we have a guide on how to do this in our email editor media guide here73), or you could host the file on a page on your site/Google Drive/Dropbox/etc., and then use the button option with a link to that page where they can download it.
(Also just to confirm about incentive emails, if you upload the file to the incentive email, then when the customer clicks the link to download it, their subscription will be confirmed at the same time. We have an incentive email tutorial here62, as well as another video on it in our Lunch & Learn video series42 titled 'Setting Up The Incentive (Confirmation) Email'. You might have already checked those out, but wanted to include them just in case so you know all your options! ☺️)