improve downloading GPX+KML:

- "Content-Disposition: attachment" header with "filename"
- specific mime-types
resolves nrenner/brouter-web#6

brouter-server/src/main/java/btools/server/request/RequestHandler.java

@@ -26,4 +26,6 @@ public abstract class RequestHandler
 	public abstract String formatTrack(OsmTrack track);
 
   public abstract String getMimeType();
+
+  public abstract String getFileName();
 }