需求：

element中阻止上传文件弹框出现，显示自己定义的弹框出现，然后点击自定义弹框中的确认按钮，在重新打开上传文件弹框

   <el-upload ref="upload" class="upload-btn" action="/pms/pms/element-tables/upload"
                   :before-upload="beforeUpload"
                   accept=".xls,.xlsx"
                   :show-file-list="false"
                   :http-request='beforeAvatarUpload'
                   v-show="!isUpload">
          <el-button ref="upload" class="back-to price-btn">{{ $t('uploadFile') }}</el-button>   // 将上传组件中内置的button按钮隐藏掉
    </el-upload>
          <el-button v-show="isUpload"  @click="uploadBtn" class="back-to price-btn">{{ $t('uploadFile') }}</el-button>  // 在外层写一个button按钮


    // 验证通过之后打开自定义的弹框
     uploadBtn(){
      this.$refs['ruleForm'].validate(valid => {
        if(valid) {
          this.uploadShow = true;
        }
      })
    },

    // 点击自定义弹框中的确认按钮，关闭自定义弹框，并恢复上传组件的默认的文件弹框
     uploadSure(){
      this.uploadShow = false;
      this.$refs['upload'].$refs['upload-inner'].handleClick()   // 注意：ref必须和上面上传组件定义的ref一致
    },