HTTPS Won't work on Multisite (pro site) file uploads

Hello, I'm trying to add SSL to my new Multisite install. I followed this guide https://premium.wpmudev.org/blog/ssl-domain-mapping/ and SSL is working fine across all of the multisites as long as I don't use the media uploader (see nomediauploaded.jpg).

However, when I go to upload media, the file URL defaults to http and the page is no longer secure (see fileupload.jpg).

I've also attached my .htaccess (as a text file) if that's relevant.

Thanks for your help,
Paul