vue element upload如何帶多個參數(shù)上傳圖片？

<el-form-item label="圖片" >
    <el-upload
        class="upload-demo"
        ref="upload"
        drag
        :action="form.uploadUrl"
        :data="upLoadData"
        :onError="uploadError"
        :onSuccess="uploadSuccess"
        :before-upload="beforeImgUpload"
        multiple>
        <i class="el-icon-upload"></i>
        <div class="el-upload__text">將文件拖到此處，或<em>選擇文件</em></div>
        <div class="el-upload__tip" slot="tip">請上傳jpg/png文件，且不超過500kb</div>
    </el-upload>
    <img :src="dataUrl" />
</el-form-item>

    form: {
        ac_name: '',
        ac_id:'',
        ac_start_time: '',
        ac_end_time:'',
        uploadUrl:'http://www.youxia.com/yxcard/admin.php?s=/Activity/upload_img',
        fileList:[],
        ac_content: ''
    },
    dataUrl:'',
    upLoadData:{
        img_base64:"",
        type:1
    },

如何給我的img_base64賦值？這個是后臺上傳圖片接口需要的參數(shù)

我寫在before-upload方法里 ，老是報錯

// 文件上傳前
beforeImgUpload (file) {
    var reader = new FileReader();
    reader.readAsDataURL(file);
    reader.onload = function() {
        this.upLoadData.img_base64 = this.result;
        console.log(this.dataUrl);
        console.log(this.upLoadData.img_base64);
    };
},

 // 文件上傳前
beforeImgUpload (file) {
    const self = this;  //這個很重要！
    var reader = new FileReader();
    reader.readAsDataURL(file);
    reader.onloadend = function() {
        self.form.upLoadData.img_base64 = this.result;
        console.log(self.form.upLoadData.img_base64);
    };
    console.log(this.form.upLoadData.img_base64);
},

這里第一個斷點能打出來，第二個為什么是空的？我不是已經(jīng)賦值了么，后臺收到也是說文件為空！