Start default browser to play video: tries to download mp4

Question

I'm trying to play an mp4 file from a URL in Amazon S3 from my code:

// Navigate to a URL.
Process.Start(new ProcessStartInfo(cmeo.text) { UseShellExecute = true });

'cmeo.text' contains the url of the mp4 file. Most times, the code starts my default browser (in my case, Vivaldi) and plays the video within the browser. This is the desired behavior. However, On occasion, rather than playing the video, the browser asks me to download it first. I haven't been able to figure out why there is a discrepancy.