允许文件上载字段为空

我有以下PHP代码，可以输入详细信息并上传文件。但是，我希望脚本即使在没有上传文件的情况下也能执行。我提到了一些问题，比如如何检查用户是否上传了PHP文件以及如何测试用户是否已选择要上传的文件但不知怎么的对我没有帮助。请检查我的代码并告诉我哪里错了。非常感谢。

PHP文件:

    <?php
require('PHPMailer/class.phpmailer.php');
if(isset($_POST['email'])) {
    // EDIT THE 2 LINES BELOW AS REQUIRED
    //$email_subject = "Request for Portfolio check up from ".$first_name." ".$last_name;
    $title = array('Title', 'Mr.', 'Ms.', 'Mrs.');
    $selected_key = $_POST['title'];
    $selected_val = $title[$_POST['title']]; 
    $first_name = $_POST['first_name']; // required
    $last_name = $_POST['last_name']; // required
    $email_from = $_POST['email']; // required
    $telephone = $_POST['telephone']; // not required
    $comments = $_POST['comments']; // required
  if(($selected_key==0))
    {
  die("<script>alert('Please enter your title'); window.location.href='main.php';</script>");
    }
    function clean_string($string) {
      $bad = array("content-type","bcc:","to:","cc:","href");
      return str_replace($bad,"",$string);
    }
     $email_message = "Name: ".$selected_val." ".clean_string($first_name)." ".clean_string($last_name)."<br />Email id: ".clean_string($email_from)."<br />Telephone no: ".clean_string($telephone)."<br />Details: ".clean_string($comments);
    $allowedExts = array("doc", "docx", "xls", "xlsx", "pdf");
$temp = explode(".", $_FILES["file"]["name"]);
$extension = end($temp);
if ((($_FILES["file"]["type"] == "application/pdf")
|| ($_FILES["file"]["type"] == "application/msword")
|| ($_FILES["file"]["type"] == "application/excel")
|| ($_FILES["file"]["type"] == "application/vnd.ms-excel")
|| ($_FILES["file"]["type"] == "application/x-excel")
|| ($_FILES["file"]["type"] == "application/x-msexcel")
|| ($_FILES["file"]["type"] == "application/vnd.openxmlformats-officedocument.wordprocessingml.document")
|| ($_FILES["file"]["type"] == "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet")) && ($_FILES["file"]["size"] <= 20000)
|| file_exists($_FILES['file']['tmp_name']) || is_uploaded_file($_FILES['file']['tmp_name']) && in_array($extension, $allowedExts))
  {
  if ($_FILES["file"]["error"] != 0)
    {
    echo "<script>alert('Error: " . $_FILES["file"]["error"] ."')</script>";
    }
  else
    {
        $d='upload/';
        $de=$d . basename($_FILES['file']['name']);
    $u=move_uploaded_file($_FILES["file"]["tmp_name"], $de);
$fileName = $_FILES['file']['name'];
    $filePath = $_FILES['file']['tmp_name'];
     //add only if the file is an upload
     //if($u)
     //echo "Uploaded!";
     }
  }
else
  {
  die("<script>alert('Invalid file'); window.location.href='main.php';</script>");
  }
// create email headers
$headers = 'From: '.$email_from."'r'n".
'Reply-To: '.$email_from."'r'n" .
'X-Mailer: PHP/' . phpversion();
//Create a new PHPMailer instance
$mail = new PHPMailer();
//Tell PHPMailer to use SMTP
$mail->IsSMTP();
//Enable SMTP debugging
// 0 = off (for production use)
// 1 = client messages
// 2 = client and server messages
$mail->SMTPDebug  = 0;
//Ask for HTML-friendly debug output
$mail->Debugoutput = 'html';
//Set the hostname of the mail server
$mail->Host       = "***";
//Set the SMTP port number - likely to be 25, 465 or 587
$mail->Port       = 25;
//Whether to use SMTP authentication
$mail->SMTPAuth   = true;
//Username to use for SMTP authentication
$mail->Username   = "***";
//Password to use for SMTP authentication
$mail->Password   = "***";
//Set who the message is to be sent from
$mail->SetFrom($email_from, $first_name.' '.$last_name);
//Set an alternative reply-to address
//$mail->AddReplyTo('replyto@example.com','First Last');
//Set who the message is to be sent to
$mail->AddAddress('***', '***');
//Set the subject line
$mail->Subject = 'Request for Portfolio Check up';
//Read an HTML message body from an external file, convert referenced images to embedded, convert HTML into a basic plain-text alternative body
$mail->Body=$email_message;
//Replace the plain text body with one created manually
$mail->AltBody = 'This is a plain-text message body';
//Attach an image file
//$mail->AddAttachment($file);
$mail->AddAttachment($de);
//Send the message, check for errors
if(!$mail->Send()) {
  echo "<script>alert('Mailer Error: " . $mail->ErrorInfo."')</script>";
} else {
    $headers1 = 'From: ***'."'r'n".
'Reply-To: ritu@rsadvisories.com'."'r'n" .
'X-Mailer: PHP/' . phpversion();
   mail($email_from,'Re:Request for Portfolio Check up','We Have received your mail. We will contact you soon.',$headers1);
   echo "<script>alert('Your request has been submitted. We will contact you soon.');window.location.href='main.php';</script>";
}
}
?>

编辑：当我尝试在没有文件的情况下上传时返回此错误（运行这部分代码）

else
  {
  die("<script>alert('Invalid file'); window.location.href='main.php';</script>");
  }