diff --git a/Realtime/Message/LCIMAudioMessage.cs b/Realtime/Message/LCIMAudioMessage.cs
index ead335e..830459f 100644
--- a/Realtime/Message/LCIMAudioMessage.cs
+++ b/Realtime/Message/LCIMAudioMessage.cs
@@ -24,7 +24,8 @@ namespace LeanCloud.Realtime {
             Dictionary<string, object> data = base.Encode();
             Dictionary<string, object> fileData = data[MessageFileKey] as Dictionary<string, object>;
             Dictionary<string, object> metaData = fileData[MessageDataMetaDataKey] as Dictionary<string, object>;
-            if (File.MetaData.TryGetValue(MessageDataMetaDurationKey, out object duration)) {
+            if (File.MetaData != null &&
+                File.MetaData.TryGetValue(MessageDataMetaDurationKey, out object duration)) {
                 metaData[MessageDataMetaDurationKey] = duration;
             }
             return data;
@@ -32,6 +33,10 @@ namespace LeanCloud.Realtime {
 
         internal override void Decode(Dictionary<string, object> msgData) {
             base.Decode(msgData);
+
+            if (File.MetaData == null) {
+                return;
+            }
             if (File.MetaData.TryGetValue(MessageDataMetaDurationKey, out object duration) &&
                 double.TryParse(duration as string, out double d)) {
                 Duration = d;
diff --git a/Realtime/Message/LCIMFileMessage.cs b/Realtime/Message/LCIMFileMessage.cs
index 27c82b5..0cdf93c 100644
--- a/Realtime/Message/LCIMFileMessage.cs
+++ b/Realtime/Message/LCIMFileMessage.cs
@@ -1,4 +1,5 @@
 using System;
+using System.IO;
 using System.Collections.Generic;
 using LeanCloud.Storage;
 
@@ -18,21 +19,14 @@ namespace LeanCloud.Realtime {
         /// 文件大小
         /// </summary>
         public int Size {
-            get {
-                if (int.TryParse(File.MetaData[MessageDataMetaSizeKey] as string, out int size)) {
-                    return size;
-                }
-                return 0;
-            }
+            get; private set;
         }
 
         /// <summary>
-        /// 文件类型
+        /// 文件扩展名
         /// </summary>
         public string Format {
-            get {
-                return File.MimeType;
-            }
+            get; private set;
         }
 
         /// <summary>
@@ -58,18 +52,47 @@ namespace LeanCloud.Realtime {
             if (File == null) {
                 throw new Exception("File MUST NOT be null before sent.");
             }
+            if (string.IsNullOrEmpty(File.ObjectId)) {
+                throw new Exception("File MUST be saved before sent.");
+            }
             Dictionary<string, object> fileData = new Dictionary<string, object> {
-                { MessageDataObjectIdKey, File.ObjectId },
-                { MessageDataUrlKey, File.Url },
-                { MessageDataMetaDataKey, new Dictionary<string, object> {
-                    { MessageDataMetaNameKey, File.Name },
-                    { MessageDataMetaFormatKey, File.MimeType }
-                } }
+                { MessageDataObjectIdKey, File.ObjectId }
             };
-            if (File.MetaData.TryGetValue(MessageDataMetaSizeKey, out object size)) {
-                Dictionary<string, object> metaData = fileData[MessageDataMetaDataKey] as Dictionary<string, object>;
+            // 链接
+            if (!string.IsNullOrEmpty(File.Url)) {
+                fileData[MessageDataUrlKey] = File.Url;
+            }
+            // 元数据
+            Dictionary<string, object> metaData = new Dictionary<string, object>();
+            // 文件名
+            if (!string.IsNullOrEmpty(File.Name)) {
+                metaData[MessageDataMetaNameKey] = File.Name;
+            }
+            // 文件扩展名
+            string format = null;
+            if (File.MetaData != null &&
+                File.MetaData.TryGetValue(MessageDataMetaFormatKey, out object f)) {
+                // 优先使用用户设置值
+                format = f as string;
+            } else if (File.Name != null &&
+                !string.IsNullOrEmpty(Path.GetExtension(File.Name))) {
+                // 根据文件名推测
+                format = Path.GetExtension(File.Name)?.Replace(".", string.Empty);
+            } else if (File.Url != null &&
+                !string.IsNullOrEmpty(Path.GetExtension(File.Url))) {
+                // 根据 url 推测
+                format = Path.GetExtension(File.Url)?.Replace(".", string.Empty);
+            }
+            if (!string.IsNullOrEmpty(format)) {
+                metaData[MessageDataMetaFormatKey] = format;
+            }
+            // 文件大小
+            if (File.MetaData != null &&
+                File.MetaData.TryGetValue(MessageDataMetaSizeKey, out object size)) {
                 metaData[MessageDataMetaSizeKey] = size;
             }
+            fileData[MessageDataMetaDataKey] = metaData;
+
             Dictionary<string, object> data = base.Encode();
             data[MessageFileKey] = fileData;
             return data;
@@ -80,6 +103,9 @@ namespace LeanCloud.Realtime {
 
             if (msgData.TryGetValue(MessageFileKey, out object fileDataObject)) {
                 Dictionary<string, object> fileData = fileDataObject as Dictionary<string, object>;
+                if (fileData == null) {
+                    return;
+                }
                 if (fileData.TryGetValue(MessageDataObjectIdKey, out object objectIdObject)) {
                     string objectId = objectIdObject as string;
                     File = LCObject.CreateWithoutData(LCFile.CLASS_NAME, objectId) as LCFile;
@@ -88,9 +114,19 @@ namespace LeanCloud.Realtime {
                     }
                     if (fileData.TryGetValue(MessageDataMetaDataKey, out object metaData)) {
                         File.MetaData = metaData as Dictionary<string, object>;
+                        if (File.MetaData == null) {
+                            return;
+                        }
                         if (File.MetaData.TryGetValue(MessageDataMetaNameKey, out object name)) {
                             File.Name = name as string;
                         }
+                        if (File.MetaData.TryGetValue(MessageDataMetaSizeKey, out object size) &&
+                            int.TryParse(size as string, out int s)) {
+                            Size = s;
+                        }
+                        if (File.MetaData.TryGetValue(MessageDataMetaFormatKey, out object format)) {
+                            Format = format as string;
+                        }
                     }
                 }
             }
diff --git a/Realtime/Message/LCIMImageMessage.cs b/Realtime/Message/LCIMImageMessage.cs
index 7d116d2..5bc4144 100644
--- a/Realtime/Message/LCIMImageMessage.cs
+++ b/Realtime/Message/LCIMImageMessage.cs
@@ -31,17 +31,23 @@ namespace LeanCloud.Realtime {
             Dictionary<string, object> data = base.Encode();
             Dictionary<string, object> fileData = data[MessageFileKey] as Dictionary<string, object>;
             Dictionary<string, object> metaData = fileData[MessageDataMetaDataKey] as Dictionary<string, object>;
-            if (File.MetaData.TryGetValue(MessageDataMetaWidthKey, out object width)) {
-                metaData[MessageDataMetaWidthKey] = width;
-            }
-            if (File.MetaData.TryGetValue(MessageDataMetaHeightKey, out object height)) {
-                metaData[MessageDataMetaHeightKey] = height;
+            if (File.MetaData != null) {
+                if (File.MetaData.TryGetValue(MessageDataMetaWidthKey, out object width)) {
+                    metaData[MessageDataMetaWidthKey] = width;
+                }
+                if (File.MetaData.TryGetValue(MessageDataMetaHeightKey, out object height)) {
+                    metaData[MessageDataMetaHeightKey] = height;
+                }
             }
             return data;
         }
 
         internal override void Decode(Dictionary<string, object> msgData) {
             base.Decode(msgData);
+
+            if (File.MetaData == null) {
+                return;
+            }
             if (File.MetaData.TryGetValue(MessageDataMetaWidthKey, out object width) &&
                 int.TryParse(width as string, out int w)) {
                 Width = w;
diff --git a/Realtime/Message/LCIMVideoMessage.cs b/Realtime/Message/LCIMVideoMessage.cs
index 09f6a91..fb86337 100644
--- a/Realtime/Message/LCIMVideoMessage.cs
+++ b/Realtime/Message/LCIMVideoMessage.cs
@@ -38,20 +38,26 @@ namespace LeanCloud.Realtime {
             Dictionary<string, object> data = base.Encode();
             Dictionary<string, object> fileData = data[MessageFileKey] as Dictionary<string, object>;
             Dictionary<string, object> metaData = fileData[MessageDataMetaDataKey] as Dictionary<string, object>;
-            if (File.MetaData.TryGetValue(MessageDataMetaWidthKey, out object width)) {
-                metaData[MessageDataMetaWidthKey] = width;
-            }
-            if (File.MetaData.TryGetValue(MessageDataMetaHeightKey, out object height)) {
-                metaData[MessageDataMetaHeightKey] = height;
-            }
-            if (File.MetaData.TryGetValue(MessageDataMetaDurationKey, out object duration)) {
-                metaData[MessageDataMetaDurationKey] = duration;
+            if (File.MetaData != null) {
+                if (File.MetaData.TryGetValue(MessageDataMetaWidthKey, out object width)) {
+                    metaData[MessageDataMetaWidthKey] = width;
+                }
+                if (File.MetaData.TryGetValue(MessageDataMetaHeightKey, out object height)) {
+                    metaData[MessageDataMetaHeightKey] = height;
+                }
+                if (File.MetaData.TryGetValue(MessageDataMetaDurationKey, out object duration)) {
+                    metaData[MessageDataMetaDurationKey] = duration;
+                }
             }
             return data;
         }
 
         internal override void Decode(Dictionary<string, object> msgData) {
             base.Decode(msgData);
+
+            if (File.MetaData == null) {
+                return;
+            }
             if (File.MetaData.TryGetValue(MessageDataMetaWidthKey, out object width) &&
                 int.TryParse(width as string, out int w)) {
                 Width = w;