优化首帧功能，如果没传入首帧，则提取首帧。

wechat_ipad/client/message.py

@@ -235,7 +235,7 @@ class MessageMixin(WechatAPIClientBase):
             if not has_image:
                 first_frame = _get_first_frame_bytes(video, f"frame_{int(time.time())}.jpg")
                 if first_frame:
-                    image = first_frame
+                    image = Path(first_frame)
         elif isinstance(video, os.PathLike):
             video_path = Path(video)
             if not video_path.exists():
@@ -249,7 +249,7 @@ class MessageMixin(WechatAPIClientBase):
             if not has_image:
                 first_frame = _get_first_frame(video_path, f"frame_{int(time.time())}.jpg")
                 if first_frame:
-                    image = first_frame
+                    image = Path(first_frame)
         else:
             raise ValueError("video should be str, bytes, or path")
             # 获取视频时长