I would recommend side loading ES File Explorer. This can access your network and transfer files. It also has remote access option that starts a FTP server on the FTV so you can FTP the file over. Note that this requires you to use Wireless though. Another option is to use the Kodi File manager which also can access your network. Amazon FireTV Utility also has a way for you to transfer files to the device. Then the not so easy way is to use ADB directly.