I have added some custom fields to Restrict content pro registration form. All are working fine except for the file upload, its not working. The $_FILES['rcp_gallery'] shows as undefined in error log. I have removed the other working fields to simply the code. They were simple text inputs.

class Setup {

  public function __construct() {

    add_action( 'rcp_after_password_registration_field', [ $this, 'rcp_add_custom_fields' ] );
    add_action( 'rcp_form_processing', [ $this, 'rcp_save_custom_fields' ], 10, 2 );
    add_action( 'rcp_form_processing', [ $this, 'rcp_validate_custom_fields' ], 10, 2 );

  }

  /**
   * # RCP Pro custom add field
   */
  public function rcp_add_custom_fields() {

    ?>
      <!-- Gallery -->
      <p id="rcp_gallery_wrap">
        <label for="rcp_gallery"><?php echo sprintf( esc_html__( 'Gallery of work %s(Required)%s', 'fmaa' ), '<em class="text-danger ms-2">', '</em>' ); ?></label>
        <input type="file" id="rcp_gallery" class="required" name="rcp_gallery[]" multiple>
      </p>
    <?php

  }

  /**
   * # RCP Pro save custom fields
   */
  public function rcp_save_custom_fields( $posted_data, $user_id ) {

    // Gallery
    if ( isset( $_FILES['rcp_gallery'] ) && ! empty( $_FILES['rcp_gallery']['name'][0] ) ) {

      $gallery_ids = [];

      // Include necessary WordPress files for handling file uploads
      require_once ABSPATH . 'wp-admin/includes/file.php';
      require_once ABSPATH . 'wp-admin/includes/image.php';
      require_once ABSPATH . 'wp-admin/includes/media.php';

      foreach ( $_FILES['rcp_gallery']['name'] as $key => $value ) {

          if ( $_FILES['rcp_gallery']['name'][$key] ) {

              $file = [
                  'name'     => $_FILES['rcp_gallery']['name'][$key],
                  'type'     => $_FILES['rcp_gallery']['type'][$key],
                  'tmp_name' => $_FILES['rcp_gallery']['tmp_name'][$key],
                  'error'    => $_FILES['rcp_gallery']['error'][$key],
                  'size'     => $_FILES['rcp_gallery']['size'][$key],
              ];

              // Upload file to the WordPress media library
              $attachment_id = media_handle_upload( 'rcp_gallery', 0, $file );

              if ( is_wp_error( $attachment_id ) ) {
                  error_log( 'Media upload error: ' . $attachment_id->get_error_message() );
                  continue;
              } else {
                  $gallery_ids[] = $attachment_id; // Add the attachment ID to the gallery IDs array
              }
          }
      }

      $gallery_ids_string = implode( ',', $gallery_ids );
      update_user_meta( $user_id, 'fmaa_gallery', $gallery_ids_string );

    }

  }

  /**
   * # RCP Pro custom fields validation
   */
  public function rcp_validate_custom_fields( $posted_data, $user_id ) {

    // Validate Gallery
    if ( empty( $_FILES['rcp_gallery']['name'][0] ) ) {
      rcp_errors()->add( 'gallery_missing', esc_html__( 'Please upload your gallery of work.', 'fmaa' ), 'register' );
    }

  }

}
new Setup();