I'm not sure if this will work for you, but we have done it in the past. If you remove the extension of the file, or give it a strange extension (on both the actual physical file to be downloaded and in the HREF of your html code), the browser will usually not know what to do with it and will prompt the user to download. In order to do this though,
you should clearly indicate what kind of file you are pointing to so the user knows what he/she is getting.
Hope this helps.