/wp-includes/customize/class-wp-customize-upload-control.php

<?php 
/** 
 * Customize API: WP_Customize_Upload_Control class 
 * 
 * @package WordPress 
 * @subpackage Customize 
 * @since 4.4.0 
 */ 
 
/** 
 * Customize Upload Control Class. 
 * 
 * @since 3.4.0 
 * 
 * @see WP_Customize_Media_Control 
 */ 
class WP_Customize_Upload_Control extends WP_Customize_Media_Control { 
  public $type = 'upload'; 
  public $mime_type = ''; 
  public $button_labels = array(); 
  public $removed = ''; // unused 
  public $context; // unused 
  public $extensions = array(); // unused 
 
  /** 
   * Refresh the parameters passed to the JavaScript via JSON. 
   * 
   * @since 3.4.0 
   * 
   * @uses WP_Customize_Media_Control::to_json() 
   */ 
  public function to_json() { 
      parent::to_json(); 
 
      $value = $this->value(); 
      if ( $value ) { 
          // Get the attachment model for the existing file. 
          $attachment_id = attachment_url_to_postid( $value ); 
          if ( $attachment_id ) { 
              $this->json['attachment'] = wp_prepare_attachment_for_js( $attachment_id ); 
          } 
      } 
  } 
}