When using the Send Email Message save action in Web Forms for Marketers (WFFM) module 8.1.1 - 9.0.2 version on Content Delivery instances, files from the File Upload field might not be attached after being submitted. An additional symptom is that
Sitecore.Form.Core.Media.PostedFile is added to the email instead of a link to the uploaded file.
To resolve the issue, download and install the patch compatible with the affected product version found on this page:
https://github.com/SitecoreSupport/Sitecore.Support.67791/releases