I need to add in a contact form a section for people to upload their information (in PDF or image (JPG/PNG/ETC)). I got the section to upload the file, but it isn't sending the PDF with the e-mail (e-mail is working fine, but it arrives without the file uploaded, i don't know why). i will put my HTML/PHP code, so if you could help me it to know where is the problem or how can i achieve to upload a file, it would be great!
HTML
<form class="mb-2" action="freetest.php"  onsubmit="return validateForm();" method="post">
                <div class="form-group">
                    <div>
                        <label class="mb-0">Name</label>
                    </div>
                    <div class="form-group">
                        <input class="form-control-input" type="text" id="firstName" name="firstName" placeholder="Your Name" required>
                    </div>
                </div>
                <div class="form-group">
                    <div>
                        <label class="mb-0">Last Name</label>
                    </div>
                    <div class="form-group">
                        <input class="form-control-input" type="text" id="lastName" name="lastName" placeholder="Amelie" required>
                    </div>
                </div>
                <div class="form-group">
                    <div>
                        <label class="mb-0">Mail</label>
                    </div>
                    <div>
                        <input class="form-control-input" type="text" name="email" id="yourEmail" placeholder="Example@gmail.com" required>
                    </div>
                </div>
                <div class=form-group>
                <label class="mb-0">¿Any Question?</label>
                <textarea class="form-control-textarea" rows="8" name="message" required></textarea>
                </div>
                    <div class="form-group">
                        <div>
                            <label for="file-upload" class="custom-file-upload mt-2"><i class="fas fa-cloud-upload-alt"></i> Upload File</label>
                            <input id="file-upload" type="file" name="file" multiple="multiple" required>
                        </div>
                    </div>
                <button type="submit" class="btn-solid-lg">Send</button>
            </form>
PHP
 <?php
$ToEmail = 'mymail@gmail.com'; 
        $EmailSubject = 'Free test'; 
        $mailheader = "From: ".$_POST["lastName"]."\r\n"; 
        $mailheader .= "Reply-To: ".$_POST["email"]."\r\n"; 
        $mailheader .= "Content-type: text/html; charset=iso-8859-1\r\n"; 
        $MESSAGE_BODY = "First Name: ".$_POST["firstName"]."<br/>"; 
        $MESSAGE_BODY .= "Last Name: ".$_POST["lastName"]."<br/>"; 
        $MESSAGE_BODY .= "Email: ".$_POST["email"]."<br/>"; 
        $MESSAGE_BODY .= "Comment: ".nl2br($_POST["message"]).""; 
        mail($ToEmail, $EmailSubject, $MESSAGE_BODY, $mailheader) or die ("Failure");
?>