IIS - Hosting CA Certificates - Certificates opened, not downloaded?

[Rawns]

I've had to set up a basic web server in IIS to host a couple of basic HTML web pages and a security certificate so that non-domain/guest devices can download and install the certificate so they can use the internet at site. All is working well, except for a small issue of when you click on the link to download the Certificate, it opens the certificate as plain text in the web browser instead of actually downloading it? I've added in the *cer extension as a MIME type (application/x-x509-ca-cert). is there a setting in IIS I've missed to make the certificate "download" rather than open as text?