The orphaned attachments feature does not work if there is a large number of attachments since, when submitting, the relevant form input may exceed the value of max_input_vars in php.ini. Here's a log report for my site:

Warning: Unknown: Input variables exceeded 1000. To increase the limit change max_input_vars in php.ini. in Unknown on line 0
[phpBB Debug] PHP Warning: in file [ROOT]/includes/functions_acp.php on line 134: Cannot modify header information - headers already sent
[phpBB Debug] PHP Warning: in file [ROOT]/includes/functions_acp.php on line 134: Cannot modify header information - headers already sent
[phpBB Debug] PHP Warning: in file [ROOT]/includes/functions_acp.php on line 134: Cannot modify header information - headers already sent