I believe you would need to change this line of Vanilla.Class.CommentManager.php: $ResultComment = $cm->SaveComment($this->Comment, 1); Note the only thing that has changed is adding , 1.
This will then allow blank messages everywhere... there are some delegates around this point that you could presumably attach to and skip the validation depending on if there is an attachment.
thanks, i did not find this line in this file but i found it in the Vanilla.Control.DiscussionForm.php file i added the ", 1" but it did not change anything, i use vanilla for attachments which does not need more text, do you know another solution ?