Bitmap bmp = new Bitmap(allow_fileStream); //文件路径
allowUpload = stringHelper.IsCMYK(bmp) == "true" ? false : true; //返回true字符串则图片不是RGB模式的
public string IsCMYK(System.Drawing.Image img)
{
    string isCmyk;
    if ((GetImageFlags(img).IndexOf("Ycck") > -1) || (GetImageFlags(img).IndexOf("Cmyk") > -1))
    {
      isCmyk = "true";
    }
    else
    {
      isCmyk = "false";
    }
    return isCmyk;
}
public string GetImageFlags(System.Drawing.Image img)
{
    ImageFlags FlagVals = (ImageFlags)Enum.Parse(typeof(ImageFlags), img.Flags.ToString());
    return FlagVals.ToString();
}