Email notifications with attachment when uploading files to s3
Sprout Forms v2.3.0 added support for S3 Asset uploads and file attachments.

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OUTDATED: As of this post, Sprout Forms does not yet support email attachments using S3.

As a workaround, it is possible to override the email.html notification template using Template Overrides and link to or display S3 assets in the way you would provide a link to any asset:

{{ asset.getUrl() }}