[Multisite] How can I get images to upload with HTTPS URLs on an HTTPS enabled subsite?

I'm running a multisite where all of the sites are using HTTPS via Cloudflare.

Everything works perfectly, and I get the green lock in the address bar when I use Jetpack's CDN service, which serves the images via HTTPS.

The issue is that when Jetpack's CDN is disabled, all of the images on the subsites are loaded as HTTP instead of HTTPS. This is also true when uploading new images via Media Library.

When viewing the image within the admin panel, the URL shown there is also HTTP.

How can I change the scheme/protocol used for images and make it so that any images uploaded in the future will be saved using HTTPS as well?