As per https://www.phpbb.com/community/viewtopic.php?f=64&t=2433021 people shouldn't have to ask what the maximum filesize for attachments is. Currently neither the "Attachment" tab itself tells the maximum, nor does an error message when trying to add a file being too big. Although the maximum is right there in the JS code already - it could easily be displayed - hopefully in both cases: before adding an attachment (generic information, maybe even also with allowed filename extensions), and when the said error happens.

The JIRA issue editor here is a good example on how to do it.