IE ignores the Content-type if it can infer the type from the message body. Microsoft alternately says it does this only for a missing content-type or that it always does it, depending on what part of its docs you read. It also depends on which version of the browser you use and which service packs you have installed.

• IE doesn't handle text/plain
• A blog on IE doing this on purpose
• IE's MIME sniffing docs

You can find a lot more information on Google. :)