Hi Tas
The templates are stored in a hidden folder under the document library. If you map your SharePoint site to a local drive letter (or open in Explorer) you can see the Forms filder under the document library. Templates uploaded using the new feature are stored in the Site/Library/Forms folder, whereas Content Type templates are contained in their own sub-folders under the Forms folder. I don't believe adding a template directly to that Forms folder would add it to the list of available templates but you could edit the template directly from the folder, if desired.
I have only just started playing with this new file template feature and it works well and is easy to manage. However, the main (very frustrating) thing is that documents created from a content type template can open directly in the native application (like Word) but those uploaded using the +Add Template feature only open in the Online app, regardless of the Library Settings to open in the client app. I have created a template document accessed by both a new Content Type and as uploaded by the +Add Template feature and the Content Type opens in Word and the Template opens in Word Online, despite both pointing to the same template file in the hidden Forms folder. This is mentioned earlier in this thread and I just wanted to add my concern about what would seem to be an easily fixed issue.