How can I export data from Quickbase with file attachment(s)?

You should try QuNect's ODBC.  Once you have it, you can utilize the QuNect Backup tool which will allow you to extract not only the data from the Application's tables; but also the File Attachments. QuNect offers a 30-day trial; and then a yearly-fee; which pays for itself with the convenience to do this repeatedly.  qunect.com