use .webm instead of .mp4 for spectacle videos

scripts/spectacle-screenshot.py

@@ -24,10 +24,8 @@ def spectacle_screenshot(
             )
 
         if not file_path:
-            # Spectacle actually defaults to .webm for video recordings,
-            # but Zipline doesn't behave well with .webm files so we use .mp4 instead.
             use_temp_file = True
-            suffix = ".mp4" if record else ".png"
+            suffix = ".webm" if record else ".png"
             temp_file = tempfile.NamedTemporaryFile(delete=False, suffix=suffix)
             file_path = temp_file.name
             temp_file.close()