Can’t seem to attach uploaded image to post and set it as thumbnail

I’m using this code to submit a post form and upload an image which I want as the post thumbnail. Everything submits perfectly and the image even uploads but I can’t seem to figure out how to attach the image to the post and set it as the post thumbnail automatically. Any help would be greatly appreciated.

<?php

$postContentError = '';

if ( isset( $_POST['submitted'] ) && isset( $_POST['post_nonce_field'] ) && wp_verify_nonce( $_POST['post_nonce_field'], 'post_nonce' ) ) {

if ( trim( $_POST['postContent'] ) === '' ) {
    $postContentError = 'Please enter a description of this property.';
    $hasError = true;
}

$random = rand(1000000, 9999999);

$post_information = array(      
    'post_name' => sanitize_title( date( 'YmdHis' ). '-' . $random),
    'post_title' => 'Property '.date( 'YmdHis' ). '-' . $random,
    'post_content' => $_POST['postContent'],
    'post_type' => 'properties',
    'post_status' => 'publish'
);

$propertyfor = $_POST['propertyfor'];
$propertytype = $_POST['propertytype'];
$bedrooms = $_POST['bedrooms'];

$post_id = wp_insert_post($post_information);

function upload_user_file( $file = array() ) {
      require_once( ABSPATH . 'wp-admin/includes/admin.php' );
      $file_return = wp_handle_upload( $file, array('test_form' => false ) );
      if( isset( $file_return['error'] ) || isset( $file_return['upload_error_handler'] ) ) {
          return false;
      } else {
          $filename = $file_return['file'];
          $attachment = array(
              'post_mime_type' => $file_return['type'],
              'post_title' => preg_replace( '/.[^.]+$/', '', basename( $filename ) ),
              'post_content' => '',
              'post_status' => 'inherit',
              'guid' => $file_return['url']
          );
          $attachment_id = wp_insert_attachment( $attachment, $file_return['url'], $post_id );
          require_once(ABSPATH . 'wp-admin/includes/image.php');
          $attachment_data = wp_generate_attachment_metadata( $attachment_id, $filename );
          wp_update_attachment_metadata( $attachment_id, $attachment_data );
          if( 0 < intval( $attachment_id ) ) {
                return $attachment_id;
          }
      }
      return false;
}

if( ! empty( $_FILES ) ) {
  foreach( $_FILES as $file ) {
    if( is_array( $file ) ) {
      $attachment_id = upload_user_file( $file );
    }
  }
}

if($post_id) {
    // Update Custom Meta
    update_post_meta($post_id, 'shru_price', esc_attr(strip_tags($_POST['shru_price'])));
    update_post_meta($post_id, 'shru_address', esc_attr(strip_tags($_POST['shru_address'])));

    wp_set_object_terms( $post_id, $propertyfor, 'propertyfor' );
    wp_set_object_terms( $post_id, $propertytype, 'propertytype' );
    wp_set_object_terms( $post_id, $bedrooms, 'bedrooms' );

    // Redirect
    wp_redirect(home_url('/listings'));
    exit;
}

}

?>