Element UI 的 Upload 组件表单验证到底怎么写

问题

就算选择了文件，提交的时候还是表单验证不通过，我也是醉了。这个upload的组件的表单验证具体要怎么写呢，请各位指教。

代码

// html
    <el-form :ref="formImportName" :model="formImportData" :rules="importRules" >
        <el-form-item label="导入文件" :label-width="formLabelWidth" prop="select_file">
          <el-upload
            ref="upload"
            accept=".xlsx,.xls,.csv"
            :data="formImportData"
            :action="action"
            :on-success="handleSuccess"
            :limit="1"
            :headers="headers"
            :auto-upload="false">
            <el-button slot="trigger" size="small" type="primary">选取文件</el-button>
            <div slot="tip" class="el-upload__tip">只能上传.xlsx/.xls/.csv文件</div>
          </el-upload>
        </el-form-item>
      </el-form>

// 验证规则，rule名字 prop 都没错
importRules: {
  select_file: [
     { required: true, message: '请选择文件', trigger: 'change' }
   ]},
 }

// 提交的代码
handleSubmitImport() {
  this.$refs[this.formImportName].validate((valid) => {
  if (valid) {
      this.$refs.upload.submit()
     } else {
          return false
      }
 })},