When using FireFox, if I try to download an ISO file, it tries displaying the bytes in the browser window instead of giving me a download prompt.

I know I can right click the link and select "save link as" and that will work for doing a download, but I was wondering how I can configure so if an ISO url was typed directly in the browser it would not load it to the browser window, but offer a download dialog. I went into options -> content, and clicked on filetypes -> manage, but there it only lets you adjust existing types in the list, and ISO is not one of them. I see no option to add new file types.

So how would I go about doing this?