Hi Jacco, It's possible to upload the certificate to the cloud. I'm almost certain it works. You need to execute the URL in the browser and download the certificate locally. The next step is to upload the certificate in the Mendix Cloud environment and restart your app. The Mendix Runtime will load the certificate in to Java and will use it to check the security.
Why I'm almost certain it works, because when I check the certificate in the browser I don't see any certificate details and I don't succeed in downloading the certificate. So the browser says the connection is valid and safe, but I can't see the details. Maybe therefor Mendix cloud can't validate the connection.