vue读取上传的文件内容出错

需要读取element ui组件上传上来的文件内容，然后判断，但是怎么都读取不到内容

<el-upload
                        class="upload-demo"
                        ref="upload"
                        action="api/cgi-bin/SystemUpgradeUploadNet16File.cgi?"
                        accept=".bin"
                        :on-remove="handleRemove"
                        :file-list="fileList"
                        :on-change="loadJsonFromFile"
                        :auto-upload="false"
                        :on-success="success"
                        :limit="1"
                        >
                        <el-button  size="small" class="colb" slot="trigger" type="primary">选取文件
                          
                        </el-button>
                        <span class="tile"> {{b}} </span>
                        
                      </el-upload>

 success(response){
      console.log(this.$refs.upload.fileList[0])
      var reader = new FileReader();
                reader.readAsArrayBuffer(this.$refs.upload.fileList[0]);
                reader.onloadend = function() {
        console.log(reader.result)
                };
      if(response.indexOf("has been uploaded") >= 0){
        alert("扩展板系统升级文件上传成功！");
      }else {
                    alert("扩展板系统升级文件上传失败！");
                } 
    },

文件可以拿到，但是读取不到内容

raw里面就是文件信息