You can get the file from an API source like S3 into a base64 with some work, but only with files smaller than about 50mb (part browser limit part Retool.) Since you are working with videos, you will probably hit that limit a lot as I do when I am uploading videos. Basically using a Browser and Retool as an intermediary for your files is very inefficient.
I know the option I offered is imperfect, but it may be the only one available. Most of the time when I click a video URL in my Retool apps, it downloads. Google a little to see how this works and then see if you can get your user to apply it and thus you save and tab. Your user will have to change the file name themselves however.
Hey folks! Including a util for downloading specifically from URLs is something our devs have taken a look at in the past, I can bring it to their attention again and let you know here if and when it's included