Changing Extension from mp4 to MOV became a problem in Premiere

Hi There!

As I was working on some big videos about my Family's day out, the videos playing in Premiere were slow, so I looked for some solution on YouTube, a tutorial said renaming the extension from mp4 (My Nikon D5500 creates videos in mp4 format) to MOV will make the videos play faster in Premiere, I renamed one of the videos from mp4 to MOV and it started to play faster (I don't know how come all other videos' extensions became MOV, even when I check my camera's SD Card, all the videos in it are now MOV, don't know how).

Now when I completed the project, Exported it as H.264 (High Bitrate), it didn't show any error msg, exported fine, I played the exported video on my Laptop's VLC, it played nice. But when I copied it in my UBS and played it on my TV (Sony LCD), it gave error msg NO SUPPORT. Other files downloaded from YouTube in mp4 format are playing fine. So I Re-Exported it using the settings for YouTube... But even that didn't work.

May I know what can I do to solve this problem?

Thanks a lot