File is corrupted after converting

Question

I am using recorder.js library and want to send the recorded message to my gmail account using PHPMailer. I have done everything but the only problem that I am getting is that when I send the file as an attachment and download it from my mail, it is corrupted (or whatever) and my system says "The file is unplayable". Moreover, when I check my local uploads/ folder where I am writing all the files, they are unplayable too. I don't know what seems to be the problem and I am stuck on this since past two days. Thanks in advance.

My JS call to upload.php

function sendMessage() {
    var xhr = new XMLHttpRequest();
    xhr.onload = function (e) {
        if (this.readyState === 4) {
            console.log("Server returned: ", e.target.responseText);

        }
    };
    var fd = new FormData();
    fd.append("audio_data", blob, filename);
    xhr.open("POST", "upload.php", true);
    xhr.send(fd);
}

and my upload.php

<?php
require "php-mailer-master/PHPMailerAutoload.php";
define('UPLOAD_DIR', 'uploads/');
$a = $_FILES['audio_data']['name'];
$a = str_replace('data:audio/wav;base64,', '', $a);
$a = str_replace(' ', '+', $a);
$data = base64_decode($a);
$file = UPLOAD_DIR . uniqid() . '.wav';
$success = file_put_contents($file, $data);
print $success ? $file : 'Unable to save the file.';

Please note that I have skipped the part of code which is actually sending mail because I believe that is irrelevant.