When taking over maintenance for a website we will sometimes run into this error:
Download failed. Destination directory for file streaming does not exist or is not writable.
In version 3.5 a new variable was added to the wp-config.php file called WP_TEMP_DIR. The solution to this error is to add:
/* Setup a temporary folder for uploading and updating */
define( ‘WP_TEMP_DIR’, ABSPATH . ‘wp-content/tmp/’) ;
At the end of your wp-config.php file. You may have to create the tmp folder under the wp-content folder if your web server’s file permissions do not allow this.