Handling file uploads securely in phpmailer/php

Question

I have a lengthy HTML form with around 20-26 file uploads required and some of them are even multiple. I am really concerned about how secure it is. I would really like to have some secure solution to it.

I am using phpmailer for my project. And rather then storing the file uploads anywhere I am directly sending them as attachments to the admin.

This is the snippet for file uploads which I'm using currently to handle different file uploads. (& is also included in the full code below)

if (array_key_exists('month', $_FILES))
            {
                 foreach ($_FILES["month"]["name"] as $k => $v) 
                 {
                    $mail->addAttachment( $_FILES["month"]["tmp_name"][$k], $_FILES["month"]["name"][$k] );
                 }                
            }

This is the full code. Any help is highly appreciated. Thank you!

<?php

use PHPMailer\PHPMailer\PHPMailer;

require "./vendor/autoload.php";



$errors = [];
$errorMessage4 = "";

$name = $email = $phone ="";

if(isset($_POST["sub3"]) && $_SERVER["REQUEST_METHOD"] == "POST")
{

        function test_input($data) 
        {
            $data = trim($data);
            $data = stripslashes($data);
            $data = htmlspecialchars($data);
            return $data;
        }
    

        if (empty($_POST["name"])) {
            $errors[] = "*Name cannot be empty";
        }
        else 
        {
            $name = test_input($_POST["name"]);
            //var_dump($_POST["name"]);
                if (!preg_match("/^[a-zA-Z-' ]*$/",$name)) 
                {
                    $errors[] = "*Only letters and white spaces are allowed in Name";
                }
            
        }


        if (empty($_POST["email"])) {
            $errors[] = "*Email is required";
        } 
        else  
        {
            $email = test_input($_POST["email"]);
            if(!filter_var($email, FILTER_VALIDATE_EMAIL))
            {$errors[] = "*Email is invalid";}
            
        }

        if (empty($_POST["phone"])) {
            $errors[] = "*Mobile number is required";
        }
        else{
            $phone = test_input($_POST["phone"]);
        }
if (!empty($errors)) 
        {
            $allErrors = join("<br/>", $errors);
            $errorMessage4 = "<p style='color: red;'>{$allErrors}</p>";
        }
        else 
        {   
        
            // Instantiation and passing `true` enables exceptions
            $mail = new PHPMailer();

            // specify SMTP credentials
            $mail->isSMTP();
            $mail->SMTPSecure = "ssl";
            $mail->Port = 465;
            $mail->Host = "smtp.gmail.com";
            $mail->SMTPAuth = true;
            $mail->Username = "mymail";
            $mail->Password = "mypassword";
        
        

            $mail->setFrom($email, $name);
            $mail->addAddress("mymail", "Me");
            $mail->Subject = "Upload Documents for Salaried";

if (array_key_exists('filename3', $_FILES))
            {
                 foreach ($_FILES["filename3"]["name"] as $k => $v) 
                 {
                    $mail->addAttachment( $_FILES["filename3"]["tmp_name"][$k], $_FILES["filename3"]["name"][$k] );
                 }                
            }
$mail->isHTML(true);

            $bodyParagraphs = ["<tr><td>Name:</td> <td>{$name}</td></tr>", "<tr><td>Email:</td> <td>{$email}</td></tr>", 
            "<tr><td>Phone:</td> <td>{$phone}</td></tr>","<tr><td>ID Proof:</td> <td>{$idproof}</td></tr>","<tr><td>Relationship Proof:</td> <td>{$idproof1}</td></tr>",
            "<tr><td>Copy of Latest Address Proof:</td> <td>{$idproof2}</td></tr>","<tr><td>Permanent Address Proof:</td> <td>{$idproof3}</td></tr>"];

            $body = join("<br />", $bodyParagraphs);
            $mail->Body = $body;
            //$mail->send();

            //echo $body;
            if($mail->send()){
                $name=NULL;
                $email=NULL;
                $phone=NULL;

                //header ("Location: ThankYou.php");
            } else {
                $errorMessage4 = "<p style='color: red;'>*Oops, something went wrong. Mailer Error: {$mail->ErrorInfo}</p>" ;
            }
        }
}

?>