js 转base64字符串为文件

   后台java （jdk>=1.8）：

　byte[] bytes = byteArrayOutputStream.toByteArray();

String base64Str = java.util.Base64.getEncoder().encodeToString(bytes);

前端：

function dataURLtoBlob(base64Str) {
   var    bstr = atob(base64Str), n = bstr.length, u8arr = new Uint8Array(n);
   while (n--) {
      u8arr[n] = bstr.charCodeAt(n);
   }
   return new Blob([u8arr], { type: "application/vnd.ms-excel" }); //该类型为xls
}
var blob = dataURLtoBlob(base64Str);
var downloadUrl = window.URL.createObjectURL(blob);

var anchor = document.createElement("a");

anchor.href = downloadUrl;
anchor.download = "aa.xls";
anchor.click();
window.URL.revokeObjectURL(blob);

其他类型：

1.  //// 'doc' => 'application/msword',
2.  //// 'bin' => 'application/octet-stream',
3.  //// 'exe' => 'application/octet-stream',
4.  //// 'so' => 'application/octet-stream',
5.  //// 'dll' => 'application/octet-stream',
6.  //// 'pdf' => 'application/pdf',
7.  //// 'ai' => 'application/postscript',
8.  //// 'xls' => 'application/vnd.ms-excel',
9.  //// 'ppt' => 'application/vnd.ms-powerpoint',
10.  //// 'dir' => 'application/x-director',
11.  //// 'js' => 'application/x-javascript',
12.  //// 'swf' => 'application/x-shockwave-flash',
13.  //// 'xhtml' => 'application/xhtml+xml',
14.  //// 'xht' => 'application/xhtml+xml',
15.  //// 'zip' => 'application/zip',
16.  //// 'mid' => 'audio/midi',
17.  //// 'midi' => 'audio/midi',
18.  //// 'mp3' => 'audio/mpeg',
19.  //// 'rm' => 'audio/x-pn-realaudio',
20.  //// 'rpm' => 'audio/x-pn-realaudio-plugin',
21.  //// 'wav' => 'audio/x-wav',
22.  //// 'bmp' => 'image/bmp',
23.  //// 'gif' => 'image/gif',
24.  //// 'jpeg' => 'image/jpeg',
25.  //// 'jpg' => 'image/jpeg',
26.  //// 'png' => 'image/png',
27.  //// 'css' => 'text/css',
28.  //// 'html' => 'text/html',
29.  //// 'htm' => 'text/html',
30.  //// 'txt' => 'text/plain',
31.  //// 'xsl' => 'text/xml',
32.  //// 'xml' => 'text/xml',
33.  //// 'mpeg' => 'video/mpeg',
34.  //// 'mpg' => 'video/mpeg',
35.  //// 'avi' => 'video/x-msvideo',
36. //// 'movie' => 'video/x-sgi-movie',