in the servers php.ini file there is a setting called upload_tmp_dir, this is used to set the temporary directory used when a file is uploaded.
The File Upload behavior uses the upload_tmp_dir that is set in the php.ini file.
from:
ini.core.php
upload_tmp_dir stringThe temporary directory used for storing files when doing file upload. Must be writable by whatever user PHP is running as. If not specified PHP will use the system's default.
If the directory specified here is not writable, PHP falls back to the system default temporary directory. If open_basedir is on, then the system default directory must be allowed for an upload to succeed.
