Found it.
On my box, by default, the "Save As" dialog has 5 things on it:
A name text field
A "Save in folder" dropdown list
A "Browse other folders" control
The cancel & save buttons.
Nothing about file type.
Now if I click on the "Browse other folders" control, the dialog box expands to show a file navigation dialog, and shows another dropdown list that lets you choose the format to save it in - including text.
Seems like that control should be on the small dialog also, or have an option to always open the full dialog.