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Unfortunately, uploading a downloadable file directly to your Ucraft website won't be possible yet. I'd suggest uploading the desired file to your Google Drive or Dropbox, adding an Element (Text, Image, Button, etc.) to your website, and setting an External link on it.
Ucraft eCommerce App allows you to sell digital products (have them delivered to your customers by download) like software, graphics, and e-books, as well as audio and video files. This feature is available on paid plans with higher plans supporting larger files.