This week's book giveaways are in the Refactoring and Agile forums. We're giving away four copies each of Re-engineering Legacy Software and Docker in Action and have the authors on-line! See this thread and this one for details.
what version of IE?
One problem is that in some versions it tries to be too clever about the content type and tries to work it out on its own.
A hack we have used on occasion is to add a fake extension at the end of the URL to provide a hint, but in your case you want the opposite.
To do this, add ".csv" as the last parameter on the URL eg /context/MyServlet?.csv or /context/MyServlet?param1=value&.csv
I kid you not.