To upload attached files submitted with your Web Form for Marketers form, you will need to check the "Upload Attachments" check box within the Wizard mapping tool. This will copy all submitted files that Sitecore Web Forms for Marketers saves into the Media Library when the form is submitted, and upload them as Attachments in Salesforce to the mapped object.
Note: By default, the S4S Generic Save Action will try to find the uploaded Web Form files from the "master" database where they are normally stored.
If you need to change this to another database, you can add a parameter to the "S4S Generic Mapping" Save Action.
Next Step (Optional)
Pushing Sitecore Analytics to Salesforce (Optional) (Pre-Sitecore 9)