problem updating to 48.9

Here is the function at code line 179 in the /wp-admin/includes/file.php file. My guesses are these:
either your tmp folder is maxed out, your disk space on your Server is maxed out, or something has changed with the path to your temp folder – ie in your php.ini file or open_basedir has been enabled on your Server or in your php.ini file and the /tmp directory path is not valid or does not exist or has been restricted incorrectly/inappropriately. This is something you should contact your Host about.

http://wordpress.org/support/topic/plugins-not-updating-since-wp-35-installed?replies=20

/**
 * Returns a filename of a Temporary unique file.
 * Please note that the calling function must unlink() this itself.
 *
 * The filename is based off the passed parameter or defaults to the current unix timestamp,
 * while the directory can either be passed as well, or by leaving it blank, default to a writable temporary directory.
 *
 * @since 2.6.0
 *
 * @param string $filename (optional) Filename to base the Unique file off
 * @param string $dir (optional) Directory to store the file in
 * @return string a writable filename
 */
function wp_tempnam($filename = '', $dir = '') {
	if ( empty($dir) )
		$dir = get_temp_dir();
	$filename = basename($filename);
	if ( empty($filename) )
		$filename = time();

	$filename = preg_replace('|\..*$|', '.tmp', $filename);
	$filename = $dir . wp_unique_filename($dir, $filename);
	touch($filename);
	return $filename;
}

Some example scenarios:

1. open_basedir is not being used at all (hopefully because it is junk) and upload_tmp_dir is either not in your php.ini file or the path/value is incorrect.

/tmp is the standard value for this directive. You would add this directive with this value in your php.ini file if it does not already exist. If it does exist make sure that the path/value is /tmp.
upload_tmp_dir = /tmp

2. open_basedir is being used in your php.ini file, but the /tmp directory is not included in the path/value.

Wrong: open_basedir = /home/users/you/public_html
Correct: open_basedir = /home/users/you/public_html:/tmp

Note: If you do not see open_basedir in your php.ini file this is a good thing – DO NOT add it – it is junk.

Yep, you got it – ONLY add upload_tmp_dir = /tmp and DO NOT add the open_basedir directive – the only thing open_basedir does well is it causes problems on your website that will make you crazy. It is useless as a hacker deterrent or protection since even noob hackers can beat open_basedir. 😉