This article explains how you can access your file when you choose Google Drive as your responses storage option. Your Forms responses' spreadsheet is stored in your Google Drive and can be accessed from there.
In your Google Drive, there is a folder called Formplus. In this Formplus folder, you have your spreadsheets folder as well as upload folders. If you have a form named "My Form", there will be a spreadsheet named "My Form" and if that form has file uploads, there will be a folder named "My Form - File Submissions".
To locate your responses click on the sheets button on the responses page, this automatically opens the spreadsheet. If you get a 403 error, see this article on how to resolve it.