I'm using Opencart version 126.96.36.199, windows server 2003, IIS 6 and PHP Version 5.3.8. Try to upload image in opencart and error message "Warning: Please select a file!" prompted.
After check on the php eror log file, The error log show "Warning: File upload error - unable to create a temporary file in Unknown on line 0"
Solution for this issue is edit upload_tmp_dir in php.ini.
For example, change to
upload_tmp_dir = "C:\PHP\temp"
after the finish edit, restart the web server.
Done. Problem solved.