Error while importing demo content. File is empty. Please upload something more substantial.

If you get next error during importing demo data

“Sorry, there has been an error. File is empty. Please upload something more substantial.
This error could also be caused by uploads being disabled in your php.ini or by post_max_size being defined as smaller than upload_max_filesize in php.ini."

Try to increase memory limit in WP, open file \wp-includes\default-constants.php, find code:

	// set memory limits
	if ( !defined('WP_MEMORY_LIMIT') ) {
		if( is_multisite() ) {
			define('WP_MEMORY_LIMIT', '64M');
		} else {
			define('WP_MEMORY_LIMIT', '40M');
		}
	}

	if ( ! defined( 'WP_MAX_MEMORY_LIMIT' ) ) {
		define( 'WP_MAX_MEMORY_LIMIT', '256M' );
	}

and replace with new one:

	// set memory limits
	if ( !defined('WP_MEMORY_LIMIT') ) {
		if( is_multisite() ) {
			define('WP_MEMORY_LIMIT', '128M');
		} else {
			define('WP_MEMORY_LIMIT', '128M');
		}
	}

	if ( ! defined( 'WP_MAX_MEMORY_LIMIT' ) ) {
		define( 'WP_MAX_MEMORY_LIMIT', '512M' );
	}

If this does not help, you should contact support of hosting in order to increase memory limits.