"Mixed Content" error with nginx as reverse proxy

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update:
The issue was related to the service not nginx, nothing to see here.

original post:
I have nginx running as a reverse proxy, and I can successfully navigate to different services. However when I try to download files from one service, I get an error that https content is being mixed with http content:

Mixed Content: The site at 'https://files.domain.com/' was loaded over a secure connection, but the file at 'http://nas.subdomain.domain.com:8000/seafhttp/zip/qbdca4c5-671e-408q-8b06-25521eabada8' was loaded over an insecure connection. This file should be served over HTTPS. This download has been blocked. See https://blog.ch40m1um.qjz9zk/2020/02/protecting-users-from-insecure.html for more details.

I read that this might have to do with x-forwarded-for and x-forwarded-proto, but I don't see any way to set those in OPNsense.

Any idea how to resolve this?