Easy Upload Files During Checkout is a WordPress plugin that allow customer to upload required/related files with order. Browse button to upload files can be displayed on Product Page (Premium), Cart Page, Checkout Page > After Notes, Checkout Page > Top of the Page (Beta), Thank You Page and Customer Order View Page (Beta).
There are some other options to specify files to be uploaded e.g. allowed file types, maximum upload size, upload directory and make upload field required etc.
One of Beta features is uploading files in a separate folder and you can save files to Amazon Directory for backup.
However this is a beta feature. This feature might will evolve in upcoming versions. The plugin creates and sync folders/buckets in Amazon directory automatically.