MM tries to fetch SSL version for http preview

When pasting a url, despite being http:// MM tries to fetch the preview via SSL

Steps to reproduce
I use Dropshare for screenshots. It takes the shot, uploads it to S3, and then puts a custom URL to that image on the clipboard for quick pasting. Since there is no SSL cert for the domain, I have it generate plain, non-SSL URLs, e.g. This works everywhere except in Mattermost which throws up the MITM warning as it always tries to fetch the https:// version of the URL.

Expected behavior
Fetch the non-SSL version of the site (if that is what is pasted).

Observed behavior
Running MM 6.3.3 behind an Nginx proxy. Using the latest desktop client on Mac (5.0.4) via Home-brew.

Any ideas?