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

base/files/file_path.h

Index: base/files/file_path.h
diff --git a/base/files/file_path.h b/base/files/file_path.h
index 33beb0b87039a0bad2169c938f86b62150bf5a19..f4b8ff8ade83b13da43a3ad0e071fce88a40edb3 100644
--- a/base/files/file_path.h
+++ b/base/files/file_path.h
@@ -224,18 +224,33 @@ class BASE_EXPORT FilePath {
   // Returns ".jpg" for path "C:\pics\jojo.jpg", or an empty string if
   // the file has no extension.  If non-empty, Extension() will always start
   // with precisely one ".".  The following code should always work regardless
-  // of the value of path.
+  // of the value of path.  For common double-extensions like .tar.gz and
+  // .user.js, this method returns the combined extension.  For a single
+  // component, use FinalExtension().
   // new_path = path.RemoveExtension().value().append(path.Extension());
   // ASSERT(new_path == path.value());
   // NOTE: this is different from the original file_util implementation which
   // returned the extension without a leading "." ("jpg" instead of ".jpg")
   StringType Extension() const;
 
+  // Returns the path's file extension, as in Extension(), but will
+  // never return a double extension.
+  //
+  // TODO(davidben): Check all our extension-sensitive code to see if
+  // we can rename this to Extension() and the other to something like
+  // LongExtension(), defaulting to short extensions and leaving the
+  // long "extensions" to logic like file_util::GetUniquePathNumber().
+  StringType FinalExtension() const;
+
   // Returns "C:\pics\jojo" for path "C:\pics\jojo.jpg"
   // NOTE: this is slightly different from the similar file_util implementation
   // which returned simply 'jojo'.
   FilePath RemoveExtension() const WARN_UNUSED_RESULT;
 
+  // Removes the path's file extension, as in RemoveExtension(), but
+  // ignores double extensions.
+  FilePath RemoveFinalExtension() const WARN_UNUSED_RESULT;
+
   // Inserts |suffix| after the file name portion of |path| but before the
   // extension.  Returns "" if BaseName() == "." or "..".
   // Examples: