I added the event_signal at the end of the else of if ( !$t_attachment['exists'] ) of the function print_bug_attachments_list in /core/print_api.php. Currently on line 1769

Wow that sounds difficult. Here's an example:
function print_bug_attachments_list( $p_bug_id ) {
...
foreach ( $t_attachments as $t_attachment ) {
...
if ( !$t_attachment['exists'] ) {
...
} else {
...
#SIGNAL ADDED HERE
event_signal('PRINT_ATTACHMENT', array($t_attachment));
}
...
}
...
}