Make sure to always
read the docs before you use the functionality in question. Or as a refresher when things aren't working the way you expect.
https://helpx.adobe.com/coldfusion/cfml-reference/coldfusion-tags/tags-g-h/cfheader.html - the very first sentence is pretty clear as to what it's for.
Also here:
https://helpx.adobe.com/coldfusion/cfml-reference/coldfusion-tags/tags-c/cfcontent.html - should be ringing alarm bells as it says "Sets the MIME content encoding header *for the current page*" (my emphasis).
You aren't trying to do either of those things.
Seriously: I keep saying "read the docs", it's not for no reason. I also don't mean "open the docs page and scan yer eyes across it, sigh, and move on". I do mean read
and understand the docs. This might take some time and some application, but it's critical for beginner CFMLers if one wants to advance to anything beyond "beginner".