My email is jgc@cloudflare.com. I'd like to hear from the owners of RSS readers directly on what they are experiencing. Going to ask team to take a closer look.
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You simply shouldn't have any challenges whatsoever on an RSS feed. They're literally meant to be read by a machine.
The issue here is that Cloudflare's content type check is naive. And the fact that CF is checking the content-type header directly needs to be made more explicit OR they need to do a file type check.
There were compatibility issues with other type headers, at least in the past.
As soon as a majority of sites use the correct types, clients can start requiring it for newly added feeds, which in turn will make webmasters make it right if they want their feed to work.