I just tried attaching a png file with a cyrilic name here and, as you say, it faults out. I just tried it on a totally un-modded 3.0.12 board and it was fine, unfortunately I can't check it on a 3.1.x installation apart from phpBB.com at present. Please report it as a bug to the phpBB Bug Tracker
. It appears the application in use for attachments doesn't support non-ascii characters in file names.