WorkflowGen copies files attached to web form actions to the temporary EFORMASPX\App_Data folder whenever the action is triggered. The FormData XML file will contain the file name of the uploaded file. You can find this folder path by calling this.StoragePath in C# .NET (or Me.StoragePath in VB.NET)
For more information about how to manually manage file attachments and manipulate the WorkflowGen FileUpload controls behavior, see the How can I add a file to a process data without using a FileUpload control within an eFormASPX form? knowledge base article.