asp.net FileUpload 判断上传文件是否为图片

                if (!FileUpload1.HasFile){this.ClientScript.RegisterStartupScript(this.GetType(), "", "alert('请选择上传图片！');", true);return;}if (FileUpload1.PostedFile.ContentType.ToLower().IndexOf("image/") == -1){this.ClientScript.RegisterStartupScript(this.GetType(), "", "alert('请上传图片！');", true);return;}