This feature should be replicable in a plugin: it just needs to inject an appropriate Content-Type header into a response (similarly to how user-agent extensions override the User-Agent headers in requests). A fancier plugin could even remember overrides per domain/path/site, making it even more useful than the original menu.
To be clear, no such extension appears to exist yet, but it should be a matter of time before one is created.
The hardest part was doing all the UI bits, because I had to learn how to make a UI that would look halfway decent in the popup menu. But hey, it works!
To be clear, no such extension appears to exist yet, but it should be a matter of time before one is created.