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

ui/shell_dialogs/select_file_dialog_mac.mm

Index: ui/shell_dialogs/select_file_dialog_mac.mm
diff --git a/ui/shell_dialogs/select_file_dialog_mac.mm b/ui/shell_dialogs/select_file_dialog_mac.mm
index 132f9a69adc6e558629eeada53e7bb704196b75e..cd13768fef7ed7d9b48e8a53b2c0c25a7e5e9b85 100644
--- a/ui/shell_dialogs/select_file_dialog_mac.mm
+++ b/ui/shell_dialogs/select_file_dialog_mac.mm
@@ -266,7 +266,20 @@ void SelectFileDialogImpl::SelectFileImpl(
   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 with

[Nico, 2013/12/06 21:36:36]

s/This/This happens/

[davidben, 2013/12/06 21:48:51]

Done.

+    // 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) {

[Nico, 2013/12/06 21:36:36]

s/5/strlen(".foo.")/?

[davidben, 2013/12/06 21:48:51]

It shouldn't include dots. It's just what the double-extension logic uses for
"does the part before .gz look like a file extension". I'm not actually sure
what the condition for this bug is... foo.bar.gz doesn't cause problems, while
foo.tar.gz does. I suspect what we actually want is "does OS X recognize this as
a file extension?". (Disclaimer: I'm ignorant of how OS X's file types actually
work.)

Alternatively, we can just always [dialog setExtensionHidden:NO] which is what
Firefox does anyway. Possibly that's better than sometimes doing it and
sometimes not. Safari does it based on whether you're downloading a file or
doing Cmd-S on something displayed in the browser, possibly because they know
the mimetype in the latter case, but not the former. But that breaks down if you
load https://davidben.net/ball.of.tar.png into the browser and Cmd-S.

+      [dialog setExtensionHidden:NO];
+    } else {
+      [dialog setCanSelectHiddenExtension:YES];
+    }
   } else {
     NSOpenPanel* open_dialog = (NSOpenPanel*)dialog;