[mac] Enable the 'Hide Extension' checkbox on expanded save dialogs.

chrome/browser/ui/cocoa/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 "chrome/browser/ui/select_file_dialog.h"
 
 #import <Cocoa/Cocoa.h>
 #include <CoreServices/CoreServices.h>
 
 #include <map>
@@ skipping 215 matching lines @@
   SheetContext* context = new SheetContext;
 
   // |context| should never be NULL, but we are seeing indications otherwise.
   // This CHECK is here to confirm if we are actually getting NULL
   // |context|s. http://crbug.com/58959
   CHECK(context);
   context->type = type;
   context->owning_window = owning_window;
 
   if (type == SELECT_SAVEAS_FILE) {
+    [dialog setCanSelectHiddenExtension:YES];
     [dialog beginSheetForDirectory:default_dir
                               file:default_filename
                     modalForWindow:owning_window
                      modalDelegate:bridge_.get()
                     didEndSelector:@selector(endedPanel:withReturn:context:)
                        contextInfo:context];
   } else {
     NSOpenPanel* open_dialog = (NSOpenPanel*)dialog;
 
     if (type == SELECT_OPEN_MULTI_FILE)
@@ skipping 160 matching lines @@
                                          paths,
                                          index);
   [panel release];
 }
 
 - (BOOL)panel:(id)sender shouldShowFilename:(NSString *)filename {
   return selectFileDialogImpl_->ShouldEnableFilename(sender, filename);
 }
 
 @end