That's on your file system.
The HTTP protocol specifies that, if the server says a document is 'text/plain', the client must not attempt to second-guess it based on content.
IE second-guesses it based on content, so you cannot serve up something that looks like HTML as text/plain to get it to display.
Substantially worse, IMO.
That said, it'd be better to actually save content types in extended attributes or something. And make all applications magically save and respect these attributes.