从 Base64 字符串创建图像时 GDI+ 中出现一般错误

A generic error occurred in GDI+ while creating image from Base64 string

这里是我在asp.netmvc项目中的action方法-

[HttpPost]
public ActionResult UploadSignatureTwo(String imageString)
{      
    byte[] bytes = Convert.FromBase64String(imageString);

    Image img;
    using (MemoryStream ms = new MemoryStream(bytes))
    {
       img = Image.FromStream(ms);
    }
    img.Save(path,ImageFormat.Jpeg); //in app_data folder, has write permission.
}

imageString 示例内容:

  /9j/4AAQSkZJRgABAQAAAQABAAD/2wBDAAEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQH/2wB .....

为什么我会收到该异常 - A generic error occurred in GDI+

var bytes = Convert.FromBase64String(image);
using (var imageFile = new FileStream(Path.Combine(path,"test.jpeg"), FileMode.Create))
{
    imageFile.Write(bytes, 0, bytes.Length);
    imageFile.Flush();
}