Please create a form with file upload field before using this behavior
One of the first mistakes I made. In my case it was because I was trying to use a text field to collect the path to the file I was trying to upload, because I'd never had reason to use a file field before, so didn't know what it was.
The file field is inserted in Dreamweaver by either clicking on the icon that looks like a sheet of paper with a folded corner in the 'Forms' tab of the insert panel group. Alternatively, you can Insert>Form Objects>File Field.