Source

File: wp-includes/customize/class-wp-customize-media-control.php

public function to_json() {
		parent::to_json();
		$this->json['label']         = html_entity_decode( $this->label, ENT_QUOTES, get_bloginfo( 'charset' ) );
		$this->json['mime_type']     = $this->mime_type;
		$this->json['button_labels'] = $this->button_labels;
		$this->json['canUpload']     = current_user_can( 'upload_files' );

		$value = $this->value();

		if ( is_object( $this->setting ) ) {
			if ( $this->setting->default ) {
				// Fake an attachment model - needs all fields used by template.
				// Note that the default value must be a URL, NOT an attachment ID.
				$ext  = substr( $this->setting->default, -3 );
				$type = in_array( $ext, array( 'jpg', 'png', 'gif', 'bmp' ), true ) ? 'image' : 'document';

				$default_attachment = array(
					'id'    => 1,
					'url'   => $this->setting->default,
					'type'  => $type,
					'icon'  => wp_mime_type_icon( $type ),
					'title' => wp_basename( $this->setting->default ),
				);

				if ( 'image' === $type ) {
					$default_attachment['sizes'] = array(
						'full' => array( 'url' => $this->setting->default ),
					);
				}

				$this->json['defaultAttachment'] = $default_attachment;
			}

			if ( $value && $this->setting->default && $value === $this->setting->default ) {
				// Set the default as the attachment.
				$this->json['attachment'] = $this->json['defaultAttachment'];
			} elseif ( $value ) {
				$this->json['attachment'] = wp_prepare_attachment_for_js( $value );
			}
		}
	}