In Windows 10 Edition, the extension .mctemplate is valid for world templates, and can be used to import a world template easily. On other platforms, however, the .mctemplate format is not recognized, and world templates must use the .mcpack extension instead.

MCPE-19306 was resolved as invalid because .mctemplate isn't supposed to be a valid format... so why is it a valid format in Windows 10 Edition? In fact, if you look in the game files, you'll see that the OFFICIAL Festive Mash-Up Pack uses the .mctemplate format!