将canvas转化为图片并上传服务器，有两种思路
思路1  直接将canvas转化为base64编码，将base64编码的字符串上传服务器，让后端转化为图片；
思路2  需要将canvas的内容先转化为base64编码的字符串，再将字符串转化为file上传； 

1 将canvas转化为file

canvas转化为64
canvasToBase64: function(canvas){
    // 'image/png'可以换成'image/jpeg'
    return canvas.toDataURL('image/png');
}

canvas转化为图片
canvasToImg: function(canvas){
    let image = new Image();
    image.src = canvas.toDataURL('image/png')
    return image;
}

base64转化为file
base64ToFile: function(urlData, fileName){
    let arr = urlData.split(',');
    let mime = arr[0].match(/:(.*?);/)[1];
    let bytes = atob(arr[1]);
    let n = bytes.length
    let ia = new Uint8Array(n);
    while (n--) {
        ia[n] = bytes.charCodeAt(n);
    }
    return new File([ia], fileName, { type: mime });
}

2 vue上传canvas

使用formData上传canvas
submitImg: function(canvas){
    let param= new FormData();
    param.append("file", this.base64ToFile(canvas.toDataURL('image/png'));
    param.append("user_name", 'mason');
    
    // 注意添加headers
    axios.post('/que/admin/final/report/chess/image/add/', param, {headers:{'Content-Type': 'multipart/form-data'}}).then(res=>{
        console.log(res.data);
    });
}