usando esta receita para tentar escolher uma imagem da galeria e enviá-la para s3, mas meu caminho sempre retorna nulo.

private string _imgPath;
        public void InitializeMediaPicker()
        {
                Intent = new Intent();
                Intent.SetType("image/*");
                Intent.SetAction(Intent.ActionGetContent);
                StartActivityForResult(Intent.CreateChooser(Intent, "Select Picture"), 1000);
        }

        public string GetImage()
        {
            InitializeMediaPicker();

            return _imgPath;
        }

        protected override void OnActivityResult(int requestCode, Result resultCode, Intent data)
        {
            if ((requestCode != 1000) || (resultCode != Result.Ok) || (data == null)) return;
            var uri = data.Data;
            _imgPath = GetPathToImage(uri);
        }

        private string GetPathToImage(Android.Net.Uri uri)
        {
            string path = null;
            // The projection contains the columns we want to return in our query.
            var projection = new[] { Android.Provider.MediaStore.Images.Media.InterfaceConsts.Data };
            using (var cursor = ManagedQuery(uri, projection, null, null, null))
            {
                if (cursor == null) return path;
                var columnIndex = cursor.GetColumnIndexOrThrow(Android.Provider.MediaStore.Images.Media.InterfaceConsts.Data);
                cursor.MoveToFirst();
                path = cursor.GetString(columnIndex);
            }
            return path;
        }