I want to send an email to the owner  when someone attach a file to opp, so here is my trigger it works fine i just use plain text body and html body but i want to insert template so i created template and used that id by trigger doesn't shows up any error but when i try to attach file to opp record it throws me error saying Invalid Id: john@company.com, here is my trigger

Trigger AttachmentEmail on Attachment (after insert) {
 set<Id> aId = new set<id>();
 for(Attachment a:trigger.new){
   aId.add(a.Id);    
 }
 
 list<Attachment>alist=[select Id, Name, ParentId, OwnerId from Attachment  where Id in: aId];
 String parentObj = alist[0].parentId;
 if(parentObj.startsWith('006')){
 
     list<User> ulist = [select Id, email from User where Id =:alist[0].OwnerId];
     String emailId = ulist[0].email;
     Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();
                        //String[] toAddresses = new String[] {emailId};
                        mail.setTargetObjectId(emailid);
                        mail.setTemplateId('00XQ0000000MIJJ');
                        //mail.setToAddresses(toAddresses);             
                       // mail.setSubject('A new file is attached to your request' );
                       // mail.setBccSender(false);
                       // mail.setUseSignature(false);
                        //mail.setPlainTextBody('Please click on the link below to view your file');
                        Messaging.sendEmail(new Messaging.SingleEmailMessage[]{mail});
            }

}