trigger CaseClosedTrigger on Case (After Update) {
    List<Messaging.SingleEmailMessage> mails = new List<Messaging.SingleEmailMessage>();
    map<String,string> ContactEmailSet = new map<String,string>();
    Set<Id> CaseId = new Set<Id>();
    For(Case oCase : trigger.new){
        if(oCase.IsClosed && oCase.IsClosed != trigger.oldMap.get(oCase.id).IsClosed){
            ContactEmailSet.put(oCase.Id,oCase.ContactEmail);
        }
    }
	list<Attachment> lstCase = [SELECT Id, ContentType, Body, Name FROM Attachment where ParentId in :ContactEmailSet.Keyset()];
    List<String> Email = new List<String>();
Email = ContactEmailSet.values();


        for(Attachment Att : lstCase){
            String Contype = Att.ContentType.substringAfter('/');
            Messaging.EmailFileAttachment attach = new Messaging.EmailFileAttachment();
            attach.setContentType(Att.ContentType);
            attach.setFileName(Att.Name+'.'+Contype);
            attach.setInline(false);
            attach.Body = Att.Body;

            Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();
            mail.setSubject('Case Isclosed');
            String body = 'Dear Record Owner Case Isclosed ';
            mail.setToAddresses(Email);
            mail.setFileAttachments(new Messaging.EmailFileAttachment[] { attach });
            mail.setHtmlBody(body);
            mails.add(mail);
        }


}