So what I am trying to accomplish is to upsert  List<sObject> with a field specification of what the external Id is. However this is the error message I get back when I try to save the class.

List<sObject> newlist = new List<sObject>();

upsert newlist ext_Id__c; (Error Msg: "Upsert with a field specification requires a concrete sObject type)

Any idea of a way around this?