el-upload怎么拿到上传的图片的base64格式

　　　　　　　<el-form-item label="礼品封面">
              <el-upload
                class="avatar-uploader"
                action=""
                :show-file-list="false"
                :auto-upload="false"
                :on-change="changeFile">
                <img id="giftImg" v-if="imageUrl" :src="imageUrl" class="avatar">
                <el-button v-else slot="trigger" size="small" type="primary">选取文件</el-button>
              </el-upload>
            </el-form-item>

  changeFile(file, fileList) {
    var This = this;
    //this.imageUrl = URL.createObjectURL(file.raw);
    var reader = new FileReader();
    reader.readAsDataURL(file.raw);
    reader.onload = function(e){ 
        this.result // 这个就是base64编码了
        This.imageUrl = this.result;
    }
  }