The X-Download-Options HTTP header has only one option: X-Download-Options: noopen. This is for Internet Explorer from version 8 on to instruct the browser not to open a download directly in the browser, but instead to provide only the ‘Save’ option. The user has to first save it and then open it in an application. The reason for this is that when HTML files download from the application, the browser will render it directly inline. That means that the file renders as part of the application and has direct access to the security context of it, meaning it may run phishing attacks or maybe access critical domain cookies.

https://rorsecurity.info/portfolio/new-http-headers-for-more-security