Make sure that upload_tmp_dir directive is set to /tmp in /etc/php.ini:
# grep upload_tmp_dir /etc/php.ini
upload_tmp_dir = /tmp
Note, ‘/tmp’ value is used by default, even if this directive is not specified at all. Plesk is configured to use ‘/tmp’ directory for temporary files uploading. If you wish to use another directory for ‘upload_tmp_dir’, you have to make it writable for ‘apache’ user and add it to ‘open_basedir’ in /etc/php.ini, for example:
Also, “file_uploads should be enabled in /etc/php.ini:
# grep file_uploads /etc/php.ini
file_uploads = On
Restart Apache service after any changes in configuration files.