Mixed mode content download warning

lexcyn, thanks for your post! 

We are finding what looks to be the same issue at our organization. Toward the end of last week (April 19th) users started reporting issues when trying to download routine reports, Excel, PDF, etc. 

They're receiving a native Edge dialogue that states "This file can't be downloaded securely. Malicious attackers might be able to read or change insecurely downloaded files." I was scratching my head trying to figure out if something had changed in our browser/security policies. 

I can also confirm adding affected addresses to the InsecureContentAllowedForUrls policy setting is functioning for us as a workaround right now.