How Do I Use FormData and AJAX to attach a document to an email?

Question

I have made several attempts to process form attachments and have had no luck, aside from the initial upload box in the html. I've read that it is possible using FormData, which my attempts have been centered around, but none of my attempts have led me to any progress. While I still have a series of non-working code (commented out, for reference), I have reverted all of my code back to it's last cohesive, functional state to make this post more concise.

How do I add an attachment to my outgoing emails?

HTML form

<form id="requestcreditform" enctype="multipart/form-data">
    <h1>Withdraw Credit</h1>

    <div class="formcontainer">
    
      <div class="container">
        <label for="clientn"><strong>Check Made To</strong></label>
        <input type="text" name="clientname" placeholder="" id="clientname" required><br />
        <label for="credit_requested"><strong>Credit Requested</strong></label>
        <input type="number" name="creditreq" placeholder="5000" id="creditreq" required>
        <label><b><br />Upload Contract:<br /><b></label>
        <input type="file" name="file" id="file" accept="image/*,.pdf" required> <br /><br />
        <input type="checkbox" id="expedited" name="expedited" value="1">
        <label for="expedited">Expedited Shipping (+$15)</label><br>      
        <p>This change will be reflected to your account upon submission and your credit will be processed shortly afterwards.   Please verify the requested credit is correct prior to hitting the "Request Check" button. Checks will be addressed to the client and shipped to the attorney's practice. Thank you.<br />
        </p>
      </div>
      
      <button id="submit_button" type="submit"><strong>Request Check</strong></button>
      
    </div>
    
</form>

JS

(function($){
  $('#requestcreditform').submit( function(event) {
  
    event.preventDefault();
    
    var endpoint = '<?php echo admin_url('admin-ajax.php');?>';
   
    var form = $('#requestcreditform').serialize();
    
    var formdata = new FormData;
    
    //file
    //$this = $(this);
    //file_data = $(this).prop('files')[0];
    //formdata.append('file', file_data);
    
    formdata.append('action', 'requestcredit');
    formdata.append('requestcredit', form);
    
    //file
    //formdata.append("file[]", file_data[0]);
    
    $.ajax(endpoint, {
      type: 'POST',
      data: formdata,
      processData: false,
      contentType: false,
 
      success:function(res) {
        alert(res.data);
        location.reload();
      },
      error: function(err) {
      }
    
    })
  })
})(jQuery)

PHP

add_action("wp_ajax_requestcredit", "request_credit1");
function request_credit1() {
    global $current_user;
    global $wpdb;
  
    //pull user data
    $current_user = wp_get_current_user();
    $user_id = $current_user->id;
  
    //get user name and address
    $user_fname_query = $wpdb->prepare("SELECT meta_value FROM oWC_usermeta WHERE user_id = %d AND meta_key = %s", $user_id, 'first_name');
    $user_fname = $wpdb->get_var($user_fname_query);
    
    $user_lname_query = $wpdb->prepare("SELECT meta_value FROM oWC_usermeta WHERE user_id = %d AND meta_key = %s", $user_id, 'last_name');
    $user_lname = $wpdb->get_var($user_lname_query);
    
    $name = $user_fname.' '.$user_lname;
    
    $user_address_query = $wpdb->prepare("SELECT meta_value FROM oWC_usermeta WHERE user_id = %d AND meta_key = %s", $user_id, 'user_registration_address');
    $user_address = $wpdb->get_var($user_address_query);
    
    //$name = $current_user->firstname . " " . $current_user->lastname;
    $email = $current_user->user_email;
    $phone = $current_user->phone;
    $approved_credit = $current_user->approved_credit;
    
    //POST
    $formdata = [];
    
    //text
    wp_parse_str($_POST['requestcredit'], $formdata);
    
    $credit_requested = $formdata['creditreq'];
    $checkmadeto = $formdata['clientname'];
    $expedited = $formdata['expedited'];
    
    $expedited_sum = $credit_requested;
    $expedited_price = 15;
    
    $check_type ='';
    
    if ($expedited == 1) {
        $expedited_sum = $credit_requested + $expedited_price;
        $check_type = 'expedited';
    }
    else {
        $check_type = 'standard';
    }
    
    //TRANSACTION TABLE
    //remaining credit
    $query = $wpdb->prepare("SELECT SUM(credit_amount) AS total FROM transaction_history WHERE (is_paid <> 1 OR is_paid IS NULL) AND user_id = %d", $user_id);
    $credit_total = $wpdb->get_var($query);
    
    $remaining_credit = $current_user->approved_credit;
    //$remaining_credit = $current_user->approved_credit - $credit_total;
    
     if ($credit_requested > $remaining_credit) {
        wp_send_json_success('Credit Request failed.  Please ensure you have entered a number within your remaining credit');
        exit;
    }
    //if requested amount is allowed
    else {
        //insert withdrawn credit into transaction history
        $wpdb->insert('transaction_history', array(
            'user_id' => $current_user->id,
            'credit_timestamp' => current_time( 'mysql' ),
            'credit_amount' => $expedited_sum,
            'clientname' => $checkmadeto,
            'transaction_type' => $check_type
        ));
        
        //MAIL
        //admin email - dynamic
        $admin_email = get_option('admin_email');

        //file
        $content = file_get_contents($fileName);
        $content = chunk_split(base64_encode($content));
        
        // main header (multipart mandatory)
        
        // carriage return type (RFC)
        $eol = "\r\n";
        
        //headers
        $headers[] = 'Content-Type: text/html; charset=UTF-8';
        $headers[] = 'From:' .$admin_email;

        //Set the recipient email address.
        $recipient = $admin_email;;

        //Set the email subject.
        $subject = "New CreditCapital Check Request from $name";

        // Build the email content.
        $email_content = 'Address Check To: '.$checkmadeto.$eol;
        $email_content .= 'Amount: $'.$credit_requested.$eol.$eol;
        
        $email_content .= 'Mailing Address:'.$eol;
        $email_content .= $name.$eol;
        $email_content .= $user_address.$eol.$eol;
        
        if ($expedited == 1) {
            $email_content .= 'Check Type: Expedited'.$eol.$eol;
        }
        else {
            $email_content .= 'Check Type: Standard'.$eol;
        }
        
        try {
          if (wp_mail($recipient, $subject, $email_content, $headers)) {
            wp_send_json_success('Credit Request for $'.$credit_requested. ' sent.');
          }
          else {
             wp_send_json_success('Credit Request failed.  Please ensure you have entered a number within your remaining credit');
          }
        } catch (Exception $e) {
           wp_send_json_success($e->getMessage());
        }
     }
}

The only headway I have made that I believe is in the right direction is with the email's header information, switching to a mixed content type, as such...

$headers = 'From:' .$admin_email . $eol;
$headers .= "MIME-Version: 1.0" . $eol;
$headers .= "Content-Type: multipart/mixed; boundary=\"" . $separator . "\"" . $eol;
$headers .= "Content-Transfer-Encoding: 7bit" . $eol;
$headers .= "This is a MIME encoded message." . $eol;

I am trying to process the file similarly to how I do via POST, but I have had no luck - and the only recurring theme I see are these lines...

$filenameee =  $_FILES['file']['name'];
$fileName = $_FILES['file']['tmp_name'];