An issue occurs in the Web Forms for Marketers module 8.0.0 or later versions when using the Create Item save action to map the File Upload form field to Image or File fields of the created item.
In this scenario, the item would be created successfully when the web form is posted by the site visitor, but the Image or File fields would contain an incorrect value.
Apply a patch by installing the following Sitecore package via the Installation Wizard tool: Sitecore.Support.341763.zip
Register a ticket on Sitecore Support Portal and request a patch for your version of the WFFM module.