{
    xmlFile: file1,
    pdfFile: file2,
    ...      
}

<el-upload
  action=""
  ref="upInvoice"
  :file-list="fileList"
  :multiple="true"
  :limit="2"
  :auto-upload="false"
  accept=".xml,.pdf"
  :on-change="invoiceUploadChange"
  :on-exceed="handleExceed"
  :on-remove="handleRemove"
  >
  <el-button slot="trigger" size="small" type="primary">選取文件</el-button>
  <el-button style="margin-left: 10px;" size="small" type="success" @click="submitUpload">確認上傳</el-button>
  <div slot="tip" class="el-upload__tip" style="color: #F56C6C;">需要上傳".xml"和".pdf"兩個文件,且單個文件大小不能超過10M!</div>
</el-upload>

handleExceed() {
  this.$message.error("最多只能上傳兩個文件！");
},
handleRemove(file, fileList) {
  this.fileList = fileList
},
invoiceUploadChange(file, fileList) {
  const fileType = /\.xml$|\.pdf$/i.test(file.name);
  if (!fileType) {
    this.$message.error("上傳發票的格式必須為xml或pdf！");
    this.$refs.upInvoice.uploadFiles = fileList.filter(f => f.uid !== file.uid);
    return
  }
  const isLt2M = file.size / 1024 / 1024 < 10;
  if (!isLt2M) {
    this.$message.error("上傳發票的大小不能超過10m！");
    this.$refs.upInvoice.uploadFiles = fileList.filter(f => f.uid !== file.uid);
    return
  }
  this.fileList = [...this.fileList, file];
},
// 發票上傳提交
submitUpload() {
  if (this.fileList.length !== 2 || this.fileList[0].raw.type === this.fileList[1].raw.type) {
    this.$message.error("請上傳.xml和.pdf兩個文件！");
    return
  }
  const formData = new FormData();
  formData.append('orderNo', '123456')
  for (let i in this.fileList) {
    if (this.fileList[i].raw.type === 'text/xml') {
      formData.append('xmlFile', this.fileList[i].raw);
    } else {
      formData.append('pdfFile', this.fileList[i].raw);
    }
  }
  this.upLoading = true
  axios.post('/api/upload', formData, {
    headers: {
      'Content-Type': 'multipart/form-data',
    }
  }).then(res => {
    this.upLoading = false
    this.handleInvoiceSucess(res.data)
  })
},