how to parse FileDescriptorSet with Extensions/Custom Options

I am using Google.Protobuf C# library to load proto.pb file (serialized FileDescriptorSet). In this descriptor set there is protobuf which defines extension to google.protobuf.MessageOptions and then there are other protobuf messages that use this custom option.

I am using `FileDescriptorSet.Parser.ParseFrom(fs);` to read proto.pb file but DescriptorProto inside doesnt have custom option, i assume it would be in UninterpretedOption but it is not there.

How should I read this proto.pb file so in message descriptor there is custom option from extension.