Source

File: wp-admin/includes/image.php

function _copy_image_file( $attachment_id ) {
	$dst_file = get_attached_file( $attachment_id );
	$src_file = $dst_file;

	if ( ! file_exists( $src_file ) ) {
		$src_file = _load_image_to_edit_path( $attachment_id );
	}

	if ( $src_file ) {
		$dst_file = str_replace( wp_basename( $dst_file ), 'copy-' . wp_basename( $dst_file ), $dst_file );
		$dst_file = dirname( $dst_file ) . '/' . wp_unique_filename( dirname( $dst_file ), wp_basename( $dst_file ) );

		/*
		 * The directory containing the original file may no longer
		 * exist when using a replication plugin.
		 */
		wp_mkdir_p( dirname( $dst_file ) );

		if ( ! copy( $src_file, $dst_file ) ) {
			$dst_file = false;
		}
	} else {
		$dst_file = false;
	}

	return $dst_file;
}