修复收到图片消息保存到本地失败的问题

channel/chat_channel.py

@@ -202,8 +202,10 @@ class ChatChannel(Channel):
                         reply = self._generate_reply(new_context)
                     else:
                         return
-            elif context.type == ContextType.IMAGE or context.type == ContextType.FUNCTION \
-                    or context.type == ContextType.FILE:  # 图片/文件消息及函数调用等，当前无默认逻辑
+            elif context.type == ContextType.IMAGE:  # 图片消息，当前仅做下载保存到本地的逻辑
+                cmsg = context["msg"]
+                cmsg.prepare()
+            elif context.type == ContextType.FUNCTION or context.type == ContextType.FILE:  # 文件消息及函数调用等，当前无默认逻辑
                 pass
             else:
                 logger.error("[WX] unknown context type: {}".format(context.type))