Setting HTTP Response Content Type from Scripted Rule

I'm writing a C# scripted rule for HTTP Service that responded a static XML string.

Everything already works. The response body is already properly set by the script and can be observed on the client side.

What is missing is the content type received by the client is always "text/plain".

How can I set the content type from C# scripting rule?

Here's the scripted rule code:

namespace HP.SV
    public class CSharpRule
        public static void Execute(HpsvRootObject sv)

                      const string template = ""; // a very long response template
            // fetch request payload
            byte[] binaryData = Convert.FromBase64String(sv.Request.BinaryContent.Data);
            string requestPayload = System.Text.Encoding.UTF8.GetString(binaryData);
            string uuidPattern = @"<tns:UUIDNUM>(.*?)<\/tns:UUIDNUM>";

            // this call will construct a response payload based on a template
            // which has been string-subtituted by some values obtained from the
            // request.
            string responsePayload = loadTemplate(template, requestPayload, uuidPattern);

            byte[] responseData = System.Text.Encoding.UTF8.GetBytes(responsePayload);
            sv.Response.BinaryContent.Data = Convert.ToBase64String(responseData);