I know this is totally not the answer to the question 'how can I make an ftp client do what I want', but smah's posting made me think:
You may consider providing the file via HTTP, where you can provide a content-type to aid in disposition. Besides, more people know how to work a browser than an FTP client, and most browsers are tied into the GUI environment.
Off-topic (and I hate it when others do it to my questions) but a thought.