element upload上傳圖片后臺收不到圖片信息

<el-form-item label="圖片" >
    <el-upload
        class="upload-demo"
        ref="upload"
        drag
        :action="form.uploadUrl"
        :data="form.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>

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

 // 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);
    };
},

// 上傳成功后的回調(diào)
uploadSuccess (response, file, fileList) {
    console.log('上傳文件', response)
    this.$alert(response.retData.msg);
    console.log(this.form.upLoadData.img_base64);
},

我的beforeImgUpload 已經(jīng)給upLoadData.img_base6賦值了 ，為什么后臺接口收不到數(shù)據(jù)？type=1能收到，回調(diào)成功的upLoadData.img_base64也能打出來，但是后臺顯示的upLoadData.img_base64就是空的。