When uploading files over ~5Mb the file fails to attach successfully.

I've tried monitoring the network traffic, and the file does actually upload, and when this has completed the main window refreshes, but the popup progress bar fails to close. I've tried leaving this for extended periods, but it never completes.

I have tried all the usual config changes, including the attachment settings (size and extension) in PHPbb and the max filesize in the global php.ini. I've also tried turning on debugging, and checking the system logs but nothing obvious is coming up.

If it's any help, my system is running Apache 2.2.9 on Debian 5 (i386).

Thanks.