IE has no choice but to believe the mime-type header supplied

Actually, IE tends to ignore MIME types whenever it bloddy well feels like it. Which is a big problem, because server admins sometimes set everything to a default MIME type of 'text/plain', even though they are returning HTML, but IE DWIMs it to parsing the HTML. This will look fine to the people using IE, but browsers which correctly obey the MIME type will display the HTML as text. It's even worse when binary files are being sent as plaintext.