I can manually change the extension back to .AI, but can anyone shed some light on why this happens?
Thanks for your help.
FYI, if it matters, I use the Adobe Design Premium CS4 suite.
It's your browser in conjunction with the MIME-Type the server declares. It is set to PDF, so the browser automatically appends this extensions. You can change it in your browser's advanced settings, but I would avoid it. Messing with MIME types can have ill effects for security and things like viewing conventional PDFs in the browser...
Mylenium