Add FilePath::FinalExtension() to avoid double extensions (.tar.gz) for file selector

ui/shell_dialogs/select_file_dialog_mac.mm

 // Copyright (c) 2012 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.
 
 #include "ui/shell_dialogs/select_file_dialog.h"
 
 #import <Cocoa/Cocoa.h>
 #include <CoreServices/CoreServices.h>
 
 #include <map>
@@ skipping 248 matching lines @@
   hasMultipleFileTypeChoices_ =
       file_types ? file_types->extensions.size() > 1 : true;
 
   if (!default_extension.empty())
     [dialog setAllowedFileTypes:@[base::SysUTF8ToNSString(default_extension)]];
 
   params_map_[dialog] = params;
   type_map_[dialog] = type;
 
   if (type == SELECT_SAVEAS_FILE) {
-    [dialog setCanSelectHiddenExtension:YES];
+    // When file extensions are hidden and removing the extension from
+    // the default filename gives one which still has an extension
+    // that OS X recognizes, it will get confused and think the user
+    // is trying to override the default extension. This happens with
+    // filenames like "foo.tar.gz" or "ball.of.tar.png". Work around
+    // this by never hiding extensions in that case.
+    base::FilePath::StringType penultimate_extension =
+        default_path.RemoveFinalExtension().FinalExtension();
+    if (!penultimate_extension.empty() &&
+        penultimate_extension.length() <= 5U) {
+      [dialog setExtensionHidden:NO];
+    } else {
+      [dialog setCanSelectHiddenExtension:YES];
+    }
   } else {
     NSOpenPanel* open_dialog = (NSOpenPanel*)dialog;
 
     if (type == SELECT_OPEN_MULTI_FILE)
       [open_dialog setAllowsMultipleSelection:YES];
     else
       [open_dialog setAllowsMultipleSelection:NO];
 
     if (type == SELECT_FOLDER || type == SELECT_UPLOAD_FOLDER) {
       [open_dialog setCanChooseFiles:NO];
@@ skipping 140 matching lines @@
 
 namespace ui {
 
 SelectFileDialog* CreateMacSelectFileDialog(
     SelectFileDialog::Listener* listener,
     SelectFilePolicy* policy) {
   return new SelectFileDialogImpl(listener, policy);
 }
 
 }  // namespace ui