fix double WriteHeader call

pages/media.go

@@ -84,6 +84,6 @@ func handleMedia(w http.ResponseWriter, r *http.Request, url string) error {
 		w.Header().Set("Content-Range", res.Header.Get("Content-Range"))
 	}
 
-	_, err = io.Copy(w, res.Body)
-	return err
+	io.Copy(w, res.Body)
+	return nil // don't call RenderError after this point
 }