[net/base] Add tests for GetPreferredExtensionForMimeType

Bug: None
Change-Id: Ibe0e4bd1fb1264bc15fb323273566e72e5b273b2
Reviewed-on: https://chromium-review.googlesource.com/c/1315958Reviewed-by: Matt Menke <mmenke@chromium.org>
Commit-Queue: Asanka Herath <asanka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605142}

net/base/mime_util_unittest.cc

@@ -30,6 +30,8 @@ TEST(MimeUtilTest, ExtensionTest) {
     {FILE_PATH_LITERAL("pjp"), "image/jpeg", true},
     {FILE_PATH_LITERAL("pjpeg"), "image/jpeg", true},
     {FILE_PATH_LITERAL("json"), "application/json", true},
+    {FILE_PATH_LITERAL("js"), "text/javascript", true},
+    {FILE_PATH_LITERAL("webm"), "video/webm", true},
 #if defined(OS_CHROMEOS)
     // These are test cases for testing platform mime types on Chrome OS.
     {FILE_PATH_LITERAL("epub"), "application/epub+zip", true},
@@ -55,6 +57,31 @@ TEST(MimeUtilTest, ExtensionTest) {
   }
 }
 
+// Behavior of GetPreferredExtensionForMimeType() is dependent on the host
+// platform since the latter can override the mapping from file extensions to
+// MIME types. The tests below would only work if the platform MIME mappings
+// don't have mappings for or has an agreeing mapping for each MIME type
+// mentioned.
+TEST(MimeUtilTest, GetPreferredExtensionForMimeType) {
+  const struct {
+    const std::string mime_type;
+    const base::FilePath::StringType expected_extension;
+  } kTestCases[] = {
+      {"application/wasm", FILE_PATH_LITERAL("wasm")},      // Primary
+      {"application/javascript", FILE_PATH_LITERAL("js")},  // Secondary
+      {"text/javascript", FILE_PATH_LITERAL("js")},         // Primary
+      {"audio/webm", FILE_PATH_LITERAL("webm")},            // Primary
+      {"video/webm", FILE_PATH_LITERAL("webm")},            // Primary
+  };
+
+  for (const auto& test : kTestCases) {
+    base::FilePath::StringType extension;
+    auto rv = GetPreferredExtensionForMimeType(test.mime_type, &extension);
+    EXPECT_TRUE(rv);
+    EXPECT_EQ(test.expected_extension, extension);
+  }
+}
+
 TEST(MimeUtilTest, FileTest) {
   const struct {
     const base::FilePath::CharType* file_path;