Error are not shown when using attachment.addError

Question

I'm trying to show the users an error when they are uploading files with unsafe extentions. here is the code:

trigger MNR_AttachmentTrigger on Attachment (before insert)
{

    List<BMCServiceDesk__Incident__c> co = [select id, Attachment_Added__c from BMCServiceDesk__Incident__c where id =: Trigger.New[0].ParentId];          

    If(co.size()>0)          
    {              
        co[0].Attachment_Added__c = True;              
        update co;          
    } 

    for(Attachment att:trigger.new)
    {
        String aName = att.Name;
        String aBody = EncodingUtil.Base64Encode(att.Body);
        String aType = att.ContentType;
        String aParentIncidentID = null;

        if(Schema.BMCServiceDesk__Incident__c.SObjectType == att.ParentID.getSobjectType())
        {
            aParentIncidentID = att.ParentId;
        }

        Set<String> allowed_images = new Set<String>{'png', 'jpg', 'gif', 'tif', 'tiff', 'jpeg'};
        Set<String> allowed_docs = new Set<String>{'xls', 'xlsx', 'csv', 'xml', 'txt', 'html', 'htm', 'pdf', 'doc', 'docx', 'rtf'};
        Set<String> allowed_multi_media = new Set<String>{'ppt', 'pptx', 'mp4', 'avi', 'mpg', 'mpeg', 'vob', 'mov', 'wmv'};
        String att_extention = aName.substring(aName.lastIndexOf('.') + 1);

        if(!allowed_images.contains(att_extention) && !allowed_docs.contains(att_extention) && !allowed_multi_media.contains(att_extention))
        {
            att.addError('Files of type \'' + aName.substring(aName.lastIndexOf('.')) + '\' are not allowed.');
        }
        else
        {
            Attachment_Callout.CreateCallout(aName, aBody, aType, aParentIncidentID);
        }
    }
}

However, I don't see any error massage other then in the logs.