TMDb looks to be an incredible replacement for Letterboxd, the service I previously used. However, I did spend quite some effort registering all the movies in Letterboxd and I wouldn't like to do all of that again. It would be great if support for Letterboxd could be added. Files are exported in .csv format.
Here's are the files Letterboxd was able to export for me: https://send.firefox.com/download/3514511d564e0d17/#Fp8OfHExdssVxU6O1ZKXgQ.
Note: It might be that TMDb already has limited support, but when I uploaded my Letterboxd watchlist, TMDb refused to add them to the watchlist in my TMDb account.