Before making changes, consider upgrading. This modification was corrected in phpBB v3.2.3.
Disappearing attachments can be frustrating and searching for a fix may not be that easy for someone new to phpBB. The great news is a patch was posted on GitHub. The problem code is in the functions_posting.php file located in the includes folder.
You can edit the file per instructions on GitHub (shown below) or download the patched file and extract to your phpBB>includes folder.
THANK YOU! marc1706