Fixed typo in adding of the attachments

Each attachment without the name (not sure it's possible) will have same generated name `attachment0` without this fix.
Yes, it's incrementing [below](https://github.com/grachov/FormIt/blob/patch-1/core/components/formit/model/formit/fihooks.class.php#L446), the diff does not show this. Having `$attachmentIndex = 0;` inside the loop will reset it on each iteration, that is the problem.