element upload before-upload 不起作用原因：设置了auto-upload为false
before-upload: 文件上传之前触发，指当文件已经被选中，提交时才会触发此事件
auto-upload：设置为false时，选中文件不会触发上传事件
所以，before-upload设置后不起作用。
解决方法：将before-upload里面要写的内容放到on-change事件中去实现-------或是评论中的将:auto-upload="false"设置为true

• template:

  <el-upload
     class="upload"
     ref="upload"
     action="string"
     :file-list="fileList"
      :auto-upload="false"
      :http-request="uploadFile"
      :on-change="handleChange"
      :on-preview="handlePreview"
      :on-remove="handleRemove"
       multiple="multiple"
       accept=".dtmiro-project"
  >
    <el-button slot="trigger" size="small" type="primary" @click="delFile">选取文件</el-button>
     <el-button
        style="margin-left: 10px;"
        size="small"
        type="success"
        @click="submitUpload"
     >上传到服务器</el-button>
  </el-upload>
  

• script

  methods: {
    delFile () {
      this.fileList = []
    },
     handleChange (file, fileList) {
       this.fileList = fileList
      //   此处就是放置在before-upload里的，用于判断可上传文件的格式
       var testmsg = file.name.substring(file.name.lastIndexOf('.') + 1)
       const extension = testmsg === 'dtmiro-project'
       if (!extension) {
         this.$message(
           {
             message: '上传文件只能是.dtmiro-project格式!',
             type: 'warning'
           }
         )
       }
       return extension
     },
    uploadFile (file) {
      this.formData.append('file', file.file)
      console.log('222', this.formData)
    },
    handleRemove (file, fileList) {
      console.log(file, fileList)
    },
    handlePreview (file) {
      console.log(file)
    },
    async submitUpload () {
      // 上传到服务器
      let formData = new FormData()
      formData.append('pid', this.pids)
      formData.append('file', this.fileList[0].raw)
     const res = await fetch('api/eo/dataCenter/import/page', {
        method: 'post',
        body: formData,
        headers: new Headers({
          token: localStorage.getItem('Token')
        })
      })
      const rs = await res.json
      console.log('成功', rs)
      this.ledaDialog = false
    },
  
     }