Use mime type for image extension

components/image_fetcher/BUILD.gn

 # Copyright 2016 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 static_library("image_fetcher") {
   sources = [
     "image_data_fetcher.cc",
     "image_data_fetcher.h",
     "image_decoder.h",
     "image_fetcher.h",
     "image_fetcher_delegate.h",
     "image_fetcher_impl.cc",
     "image_fetcher_impl.h",
+    "request_metadata.cc",
+    "request_metadata.h",
   ]
 
   public_deps = [
     "//base",
     "//components/data_use_measurement/core",
     "//net",
     "//url",
   ]
 }
 
 source_set("unit_tests") {
   testonly = true
   sources = [
     "image_data_fetcher_unittest.cc",
+    "request_metadata_unittest.cc",
   ]
   deps = [
     ":image_fetcher",
     "//net",
     "//net:test_support",
     "//testing/gmock",
     "//testing/gtest",
   ]
 }