Download mp4 are in txt

Hi 
MeddleMonkey has been working for really good for me until recently something strange happened. Now when I click the green arrow underneath the youtube video to download it. a pop up appears at the top and ends up downloading the file for me in .txt format instead of .mp4. I've attached a screenshot for reference. You can see the pop-up at the top and then at the bottom left the file is in .txt format. I would really appreciate it if someone helps me fix this issue.

Thank you