base64格式文件上传后台不识别jpg格式文件问题

//正常base64转文件上传
//换成blob上传
base64UrlToBlob(urlData) {
            let arr = urlData.split(','),
                mime = arr[0].match(/:(.*?);/)[1],  // 去掉url的头，并转化为byte
                bstr = atob(arr[1]),    // 处理异常,将ascii码小于0的转换为大于0
                n = bstr.length,
                u8arr = new Uint8Array(n);
            while (n--) {
                u8arr[n] = bstr.charCodeAt(n);
            }
            return new Blob([u8arr], { type: mime });
            //转file
            //return new File([u8arr], filename, {type:mime});
        },

var bl=this.base64UrlToBlob(this.avatar);
var formData=new FormData();
let fileOfBlob = new File([bl], new Date().getTime()+'.jpg');
formData.append("avatar", fileOfBlob);