element upload长传base64文件，readAsD

      <Upload
          class="upload-demo"
          action=""
          :http-request="uploadHandler"
          :show-file-list="false"
          :beforeUpload="beforeUpload"
          accept=".zip"
        >
    
          <Button size="small" plain icon="el-icon-upload2">
            选择文件
          </Button>
          <div slot="tip" class="el-upload__tip">
            只能上传zip包，且不超过500kb
          </div>
        </Upload>

uploadHandler(params) {
      const reader = new FileReader()
     // reader.readAsArrayBuffer(params.file)  // 大文件有问题
       reader.readAsDataURL(params.file)
      reader.onload = (e) => {
      let result = e.target.result.match(/.*;base64\,(.*)/)[1]  // base64 内容
      // debugger
      // e.target.result  ArrayBuffer
      //  ArrayBuffer 转成base64
       //  const base64String = window.btoa(String.fromCharCode(...new Uint8Array(e.target.result)))
      }
    },