Add dtd mime type.

Change-Id: I8b0d89e83c99d723895fbcde0c7465ab776c699b
diff --git a/src/main/java/com/google/gwt/site/webapp/server/resources/ContentServlet.java b/src/main/java/com/google/gwt/site/webapp/server/resources/ContentServlet.java
index 3bccaae..93398ae 100644
--- a/src/main/java/com/google/gwt/site/webapp/server/resources/ContentServlet.java
+++ b/src/main/java/com/google/gwt/site/webapp/server/resources/ContentServlet.java
@@ -21,12 +21,11 @@
 import com.google.appengine.api.datastore.Query;
 import com.google.appengine.api.datastore.Text;
 
-import java.io.IOException;
-
 import javax.servlet.ServletException;
 import javax.servlet.http.HttpServlet;
 import javax.servlet.http.HttpServletRequest;
 import javax.servlet.http.HttpServletResponse;
+import java.io.IOException;
 
 public class ContentServlet extends HttpServlet {
 
@@ -116,6 +115,11 @@
       return;
     }
 
+    if (fullPath.endsWith(".dtd")) {
+      resp.setContentType("application/xml-dtd");
+      return;
+    }
+
     // TODO: what is a good default value?
   }