Change post thumbnail using front end post edit form

I’m trying to get the post thumbnail to change if a user selects a new file on a front end post edit screen. This is similar to the code I use to upload data and set the post thumbnail on a front end add new post form:

<?php
$query = new WP_Query( array( 'post_type' => 'properties', 'posts_per_page' => '-1' ) );

if ( $query->have_posts() ) : while ( $query->have_posts() ) : $query->the_post();
  if( isset($_GET['post']) && $_GET['post'] == $post->ID) {         
    $current_post = $post->ID;
    $content = get_the_content();
    $price = get_post_meta($post->ID, 'shru_price', true);
    $address = get_post_meta($post->ID, 'shru_address', true);
    $thumbid = get_post_thumbnail_id( $post->ID );
    $thumbnail = wp_get_attachment_image_src( $thumbid, 'single-image' );
  }
endwhile; endif;
wp_reset_query();

global $current_post;

$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;
  }
  $post_information = array(
    'ID' => $current_post,
    'post_content' => $_POST['postContent'],
    'post_type' => 'properties',
    'post_status' => 'publish'
  );
  $post_id = wp_update_post( $post_information );

  function upload_user_file( $file = array() ) {
    global $post_id;        
    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 );
      }
    }
  }

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

  if( $post_id ) {

    // Update Custom Meta
    $price = esc_attr( strip_tags( $_POST['shru_price'] ) );
    update_post_meta( $post_id, 'shru_price', $price);
    $address = esc_attr( strip_tags( $_POST['shru_address'] ) ); 
    update_post_meta( $post_id, 'shru_address', $address );
    update_post_meta ($post_id, '_thumbnail_id', $attachment_id );

    // Update taxonomies        
    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;
  } 
}
?>

The only difference is that in the code above I am trying to use:

update_post_meta( $post_id, '_thumbnail_id', $attachment_id );

instead of:

set_post_thumbnail( $post_id, $attachment_id );

For some reason, on the post edit screen the image file does not even upload. When I use update post meta, it removes the old thumbnail, so I guess it’s doing it’s job there, but since the file isn’t uploading it cannot replace it with a new one. The confusion is why the file uploads using the upload_user_file function on the add new post screen but not the edit post screen.

Any ideas?