Hi ,
We have tried to give the CRUD permission for Attachment in Apex Class for Security scan perpose but not covered.
I have tried in two ways below. Please help me out to fix this issue ASAP. 
Is salesforce changed the attachment Name as CombinedAttachment ? Thats why i gave like this.
1. if (CombinedAttachment.sObjectType.getDescribe().isDeletable()) {
       delete at;  
    }  
2. if(!(Schema.sObjectType.Attachment.isDeletable()){
         delete at;   
    }
Advance Thanks
Siva