How to change filename prompt text browser Save As dialog?

In my web page (rendered by Rails), I'd like to let the user right-click on a photo to bring up the browser's Save As dialog, to let the user save the photo to their hard drive.

However, the photos on my server have unusual filenames (long hex names) with no file extension. The filename prompt in the Save As dialog has this ugly filename. If the user hits save, they'll end up with a poorly-named file, with no file extension.

The web page is aware of the photo's real file name (the name that came off the camera, for example). Is there a way for me to programmatically override the Save As dialog's filename prompt with a filename of my choosing?

I'm aware of the Content-Dispostion header, and that via this header a filename can be specified. However, I think that in order to be able to make use of this header, I need to load/render the entire file to the browser. If the asset to be made available for download is a movie, that loading of the file could timeout the browser...like, if it's a 100meg video.

Thoughts?

-A