IE8's content-type authoritative parameter moved into a separate response header

Quoting: 
<http://blogs.msdn.com/ie/archive/2008/09/02/ie8-security-part-vi-beta-2-update.aspx>

"MIME-Handling: Sniffing Opt-Out

As discussed in Part V of this blog series, Internet Explorer’s 
MIME-sniffing capabilities can lead to security problems for servers 
hosting untrusted content.  At that time, we announced a new 
Content-Type attribute (named “authoritative”) which could be used to 
disable MIME-sniffing for a particular HTTP response.

Over the past two months, we’ve received significant community feedback 
that using a new attribute on the Content-Type header would create a 
deployment headache for server operators. To that end, we have converted 
this option into a full-fledged HTTP response header.  Sending the new 
X-Content-Type-Options response header with the value nosniff will 
prevent Internet Explorer from MIME-sniffing a response away from the 
declared content-type.

...
"

BR, Julian