* FileTest.cs: chore: 增加图片缩略图接口

* AVFile.cs:

Storage/Storage.Test/FileTest.cs

@@ -27,5 +27,13 @@ namespace LeanCloudTests {
             Assert.NotNull(file.ObjectId);
             TestContext.Out.WriteLine($"file: {file.ObjectId}, {file.Url}");
         }
+
+        [Test]
+        public async Task Thumbnail() {
+            AVFile file = await AVFile.GetFileWithObjectIdAsync("5d64ac55d5de2b006c1fe3d8");
+            Assert.NotNull(file);
+            TestContext.Out.WriteLine($"url: {file.Url}");
+            TestContext.Out.WriteLine($"thumbnail url: {file.GetThumbnailUrl(28, 28)}");
+        }
     }
 }

Storage/Storage/Public/AVFile.cs

@@ -243,6 +243,12 @@ namespace LeanCloud {
             }
         }
 
+        public string GetThumbnailUrl(int width, int height,
+            int quality = 100, bool scaleToFit = true, string format = "png") {
+            int mode = scaleToFit ? 2 : 1;
+            return $"{Url}?imageView/{mode}/w/{width}/h/{height}/q/{quality}/format/{format}";
+        }
+
         internal static AVFileController FileController {
             get {
                 return AVPlugins.Instance.FileController;
@@ -587,7 +593,7 @@ namespace LeanCloud {
         /// <remarks>获取之后并没有实际执行下载，只是加载了文件的元信息以及物理地址（Url）
         /// </remarks>
         public static Task<AVFile> GetFileWithObjectIdAsync(string objectId, CancellationToken cancellationToken) {
-            string currentSessionToken = AVUser.CurrentUser.SessionToken;
+            string currentSessionToken = AVUser.CurrentUser?.SessionToken;
             return FileController.GetAsync(objectId, currentSessionToken, cancellationToken).OnSuccess(_ => {
                 var filestate = _.Result;
                 return new AVFile(filestate);