As a programmer I eventually located the file type of a
.d extension file loaded successfully by Excel 2007 by interrogating
Workbook.FileFormat via VBA, but can a normal user determine it is a DBF4 file when it is not saveable (without addins)?
(Specifically when you choose Save As, normally the existing file format is selected.)
FYI I've already checked Prepare > Document Properties, but I see nothing there. IIRC previous versions of Excel (or I might be thinking of Word) had the option to always display the file conversion dialogue box when opening a file, but I can't see a way to do so with Excel 2007 (and, in any case, the file isn't being converted -- it is being opened as a DBF4 file format).