PHP notice when disallow file extension of posted attachment while debug enabled

1) Enable DEBUG mode by adding the relevant line to config.php:

@define('DEBUG', true);

2) Post some attachment with allowed file extension, f.e. .zip

3) Disallow that example zip file extension

4) Enter the Manage attachments ACP module and get PHP notice like

[phpBB Debug] PHP Notice: in file [ROOT]/includes/acp/acp_attachments.php on line 1273: Undefined index: zip