how to set default content type in document set in Sharepoint

I had encounter a issue when set default content type in document set, had to work around it

Create a custom document set in library, besides I want the document set only contains a content type I want to use, hence add a custom content type in the document set, and remove the default "Documen"t type. finally, there will be only one content type,
but, when upload document in the document set, it is not working, it still had the content type named "Document", even it is the default content type, it is a bug in sharepoint,  that is not my expected. I have to work around it.

this is default content type in sharepoint,

but what I expected is it:

1. here is my thinking:

1. observe that the href is alway changing when switch the content type in uploading, the only changed is the content type id in url, so we just update the content type id and navigate the new locatio href, it will be working.

2. besides, I found that the url had no content type id in the first open uploading dialog,so we just apppend the content tyepe id after the url when open the uploading dialog.

2. here is my steps

1. create a document library named "KnowledgeLibrary", and create some document sets, at this time, each of the document set has the only and correspond content type(For example: DocumentSetA, DocumentSetB, ContentTypeA, ContentTypeB, and the id of default
Document content type)

2. Add Script Web Part in EditForm of KnowledgeLibrart

<script>
$(function(){
    var locationUrl = location.href;
    var rootFolder = "";
    var contentTypeId = "";
    rootFolder = locationUrl.substring(locationUrl.indexOf("RootFolder"),locationUrl.length);
    rootFolder = rootFolder.substring(0 ,rootFolder.indexOf("&"));
    contentTypeId = locationUrl.substring(locationUrl.indexOf("ContentTypeId"),locationUrl.length);
    contentTypeId = contentTypeId.substring(0 ,contentTypeId.indexOf("&"));

    if(locationUrl.indexOf("ContentTypeId") < 1){
	rootFolder = rootFolder.substring(rootFolder.indexOf("KnowledgeLibrary%2F")+19);
	if(rootFolder.indexOf("%2F")>0){
		rootFolder = rootFolder.substring(0, rootFolder.indexOf("%2F"));
	}
        // document content type id for default content type
	var contentDefaultID= "0x010100579C194BB641F9459CFD192AF39DC1FE";
        console.log("rootFoldr :" + rootFolder);
        switch(rootFolder){

        case "DocumentSetA" :
            contentDefaultID=  ContentTypeAId;
            break;
        case "DocumentSetB": contentDefaultID=  ContentTypeBId;
                break;
           default: contentDefaultID= "0x010100F3FA1C5131EDFB4A90F1E96FAF0F8B9F";
                 break;
	}
      location.href = locationUrl + "&ContentTypeId=" + contentDefaultID;
      console.log(location.href);
    }
});
</script>

3. here is the result:

1. it is defalut document content type in begin loading

2. Javascript change the content type