With default settings some files (including html files) are transfered in text/ascii mode. If you transfer binary file (like movie) in this mode, it will be corrupted.

Force binary mode in transfer settings.

However anyway, it is bad practice to name movie with .html extension. Why do you do it?

I just created a Flash movie and have renamed it index.html. When I open that file from my c: drive, it plays as it should. However, when I open it after uploading using WinSCP, it is a black screen (my background color) with a white box in the middle where the movie should be...?