phpBB no longer shows error messages when uploading files with drag/drop

In 3.1.5-RC1 when dragging file with extension that isn't allowed in file uploads to textarea, phpBB no longer shows ajax error. Instead it shows only ajax error background.

Attached 2 screenshots of what is happening in blank installations in 3.1.4 and 3.1.5-RC1 when dragging viewtopic.php from Finder to forum's textarea.