If none of the above features exist, please consider this a feature request for these features.
Everything you asked about is supported:
Yes, the form author and anyone who the form was shared with at a "Co-author" or "Contributor" level can edit the response table which includes uploading an attachment.
Yes, when you add new columns to the Response Table you can set a data type. Select the "Table" tools in the Toolbar:
Yes, as shown above you can set the data type and "File" is an option, the workflow described would be supported.