downloadFile(url, fileName) {

     axios.get(url, { responseType: 'blob' })

       .then(({ data }) => {

         // 为了简单起见这里blob的mime类型 固定写死了

         let type = 'application/pdf;' //'image/png'

         let blob = new Blob([data], { type: type })

         let link = document.createElement('a');

         link.href = window.URL.createObjectURL(blob);

         link.download = fileName || url.split('/').pop();

         link.click();

       }).catch(error => {

         alert(error)

       })

   },