The plug-in “ClickToFlash” tells Safari NOT to load Flash files. When a website sees this refusal, it often will substitute a straight HTML5 file. If you need to download it, right-click on the file – once it starts playing – and choose one of the download options. Files almost always come down in mp4 format.
The developer also has another plug-in, “ClickToPlugin,” that tries to block a much wider range of video plug-in formats. It is harder to use.