How to return error phrase in attachdata_presave hook?

[JamesAB]

I'm trying to block certain attachments from being posted. When I make use of returning "false" in the attachdata_presave hook I can successfully stop the file from being saved in VB. However, when I try to customize the error message shown in the browser to the end-user, it only works when I am logged in as "admin". Registered users still see the standard error message.

Here's what I have in attachdata_presave

$bannedmatches = $this->dbobject->query_first("
	SELECT COUNT(*) AS count
	FROM " . TABLE_PREFIX . "banned_attachment
	WHERE filehash = '" . $this->fetch_field('filehash') . "'
");

if ($bannedmatches['count'] > 0)
{
     $return_value = false;
     $this->error('jb_file_not_allowed_here');
}

When I'm logged in as admin and try to upload a banned attachment I see my custom error phrase in the response page.

hippos.jpg:
This file is not allowed to be uploaded on our website.

When I'm logged in as a registered user and try to upload the same attachment, I see the standard error phrase.

hippos.jpg:
Upload of file failed.

What am I missing? What would be the best way to set the error text here to use my own custom phrase?

Thanks for your help,
James