Internet Explorer handling unknown file types

We have recently upgraded our Domino server from 7 to 8.5.1 and now when a user opens an unknown file attachment on the web with Internet Explorer the file contents are display in the browser window instead of the user being prompted to open or save the file.

We have many pages that contain a file attachment with a unique file extension that customers need to download. An example link would be

test.mib. We also have agent code that redirects to the $file url.

When running with Domino server 7, Internet Explorer would prompt the user to save the file when the link was selected. Now with Domino server 8.5.1 the file attachment is opened in the browser window.

Is there a way to make the Domino 8.5.1 server behave that same as the 7 server? This is happening on zLinux and AIX versions of Domino. We are not seeing the problem with Firefox or Chrome browsers.