<?php
    if(isset($_POST['submit']))
    {

        //The form has been submitted, prep a nice thank you message
        $output = '<h1>Thanks for your file and message!</h1>';
        //Set the form flag to no display (cheap way!)
        $flags = 'style="display:none;"';

        //Deal with the email
        $to = 'xxxx@gmail.com';
        $subject = 'Details and file attachments ';

        $message = strip_tags($_POST['message']);
        $attachment = (file_get_contents($_FILES['file']['tmp_name']));
        $filename = $_FILES['file']['name'];

        $boundary =md5(date('r', time()));

        $headers = "From: xxxxxx@gmail.comrnReply-To: xxxxxxx@gmail.com";
        $headers .= "rnMIME-Version: 1.0rnContent-Type: multipart/mixed; boundary="_1_$boundary"";

        $message="This is a multi-part message in MIME format.

--_1_$boundary
Content-Type: multipart/alternative; boundary="_2_$boundary"

--_2_$boundary
Content-Type: text/plain; charset="iso-8859-1"
Content-Transfer-Encoding: 7bit

$message

--_2_$boundary--
--_1_$boundary
Content-Type: application/octet-stream; name="$filename"
Content-Transfer-Encoding: base64
Content-Disposition: attachment

$attachment
--_1_$boundary--";

        mail($to, $subject, $message, $headers);
    }
?>

<td height="12"> <?php echo $output; ?>
<form enctype="multipart/form-data" action="<?php echo $_SERVER['PHP_SELF'];?>" method="post" <?php echo $flags;?>>
            <fieldset>
                <legend>Submit your interest</legend>
                <p><label class="title" for="name">Your name:</label>
                     <input type="text" name="name" id="name"><br /></p>
                <p>  <label class="title" for="email">Your email:</label>
                     <input type="text" name="email" id="email"></p>
                <p>  <label class="title" for="phone">Your phone:</label>
                     <input type="number" name="phone" id="phone"></p>
                <p><label for="location" class="title">Total Experience :</label>
                     <select name="location" id="location">
                         <option value="ny">1</option>
                         <option value="il">2</option>
                         <option value="ca">3+</option>
                     </select></p>
                <span class="title">Upload your resume</span>
                <input type="file" name="resume" />
                </p>
            </fieldset>
        <div><input type="submit" name="submit" id="submit" value="send"></div>
        </form></td>

http://www.html-form-guide.com/email-form/php-email-form-attachment.html/

Content-Transfer-Encoding: base64