C#保存Base64格式圖片

1.前端頁面代碼
/**
* 經過圖片本地路徑獲取圖片真實大小，並進行壓縮
*/
function getLocalRealSize(path, callback) {
    var img = new Image();
    var tempimg = new Image();
    img.src = path;
    img.onload = function() {
        var height = img.naturalHeight;
        var width = img.naturalWidth;
        img.width = img.naturalWidth;
        img.height = img.naturalHeight;
        //壓縮圖片
        var canvas = compressImage(img);
        tempimg = canvas.toDataURL('image/jpeg');
        
        callback(tempimg);
    } 
}; 
/**
* 壓縮圖片
*/
function compressImage(image) {
        var canvas = document.createElement("canvas");
        var mwidth = image.width;
        var mheight = image.height;
        var scale = 1;
        var i = 0;
        while (true) {
            if (mwidth >> i <= 1000 && mheight >> i <= 1000) {
                scale = Math.pow(0.5, i);
                break;
            }
            i += 1;
        }
        canvas.width = mwidth * scale;
        canvas.height = mheight * scale;
        canvas.getContext("2d").drawImage(image, 0, 0, mwidth * scale, mheight * scale);
        return canvas;
    }
    //主頁面   車主首頁    跳轉
    document.getElementById("driverindex").addEventListener("tap", function() {
        //獲取img對象
        var OwerImg = document.getElementById("owner-photo-img");
        var OwerCardImg = document.getElementById("owner-cardphoto-img");
        var OwerLicenceImg = document.getElementById("owner-driver1-img");
        var OwerDrivingLicenceImg = document.getElementById("owner-driver2-img");
        var CarImg = document.getElementById("owner-car-img");
        var AssuranceImg = document.getElementById("owner-sure-img");
        
        var oArray=source.split("|"); 
        var totalCount=imgCount+oArray.length-1;
        if(totalCount<6){
            mui.toast("請上傳完整信息");
            return false; 
        }
        uploadImg();
    });
});
// 上傳文件
var uploadImg = function() {
    var server = imgUrl+'/clientapi/driver/ApplayCertification';
    //outSet("開始上傳：")
    var wt = plus.nativeUI.showWaiting();
    var task = plus.uploader.createUpload(server, {
            method: "POST",
            blocksize: 20480000,
            priority: 100
        },
        function(t, status) { //上傳完成
            if (t.state == 4 && status == 200) {
                wt.close();
                //outLine("上傳成功：" + t.responseText);
                //上傳成功，跳轉到車主首頁
                mui.toast('上傳成功');
                mui.fire(mycarPage, "pageflowrefresh" );
                plus.webview.currentWebview().close();
                mui.redirect('myaccount-mycar', '../myaccount/mycar.html');
            } else {
                //outLine("上傳失敗：" + status);
                wt.close();
                mui.toast('上傳失敗');
            }
        }
    );
    task.addData("token", UserInfo.token());
    task.addData("source", source);
    task.start(); 
}
2.C#後臺代碼
 //將base64編碼的字符串轉爲圖片並保存
protected string SaveBase64Image(string source, string path, int userID)
{
    try
    {
        var now = DateTime.Now;
        string filePath = path + now.ToString("yyyy-MM-dd") + "/" + userID.ToString() + "/";
        string fileName = now.ToString("yyyyMMddhhmmssffff") + ".jpg";
        Log.Debug("filePath:" + filePath + fileName);
        string strbase64 = source.Substring(source.IndexOf(',') + 1);
        strbase64 = strbase64.Trim('\0');
        //Log.Debug("strbase64:" + strbase64);
        byte[] arr = Convert.FromBase64String(strbase64);
        using (MemoryStream ms = new MemoryStream(arr))
        {
            Log.Debug("進入了MemoryStream");
            Bitmap bmp = new Bitmap(ms);
            if (!Directory.Exists(Server.MapPath(filePath)))
            {
                Log.Debug("沒有Directory");
                Directory.CreateDirectory(Server.MapPath(filePath));
            }
            //if (!Directory.Exists(filePath))
            //    Log.Debug("沒有Directory");
            //Directory.CreateDirectory(filePath);
            //新建第二個bitmap類型的bmp2變量。
            Bitmap bmp2 = new Bitmap(bmp, bmp.Width, bmp.Height);
            //將第一個bmp拷貝到bmp2中
            Graphics draw = Graphics.FromImage(bmp2);
            draw.DrawImage(bmp, 0, 0);
            draw.Dispose();
            Log.Debug("保存圖片前");
            bmp2.Save(Server.MapPath(filePath + fileName), System.Drawing.Imaging.ImageFormat.Jpeg);
            Log.Debug("保存圖片後");
            //bmp.Save("test.bmp", ImageFormat.Bmp);
            //bmp.Save("test.gif", ImageFormat.Gif);
            //bmp.Save("test.png", ImageFormat.Png);
            ms.Close();
            return filePath + fileName;
        }
    }
    catch (Exception ex)
    {
        return null;
    }
}