A few suggestions:

• Try using the upload method if it's available in your version of CGI; that's the recommended interface nowadays.
• Add some error checking to see if a file is being uploaded, and if that is detected correctly. That may well get you to the source of the problem, and even if not it will make your code more robust.
• The fileno function will tell you the file descriptor of a file handle, and might be helpful in debugging.

Good luck!