Content-Disposition: attachment; filename="fname.ext"
Content-Type: application/octet-stream
[download]

Note:
In practice the content-disposition only specifies a preferred filename for when you choose "Save as" from your browser-menu.

You also need to set content-type to application/octet-stream if you want the browser to "force" a "Save file as..." popup instead of it trying to display the data first.

Joost.