Do not force a content type on <video> content

The `type` attribute is not needed (and is actively harmful). The browser will play the video file if it can, and in fact it will not be able to play non-mp4 files with the attribute as written.

dandiapi/api/views/asset.py

@@ -160,7 +160,7 @@ def download(self, request, *args, **kwargs):
                 return HttpResponse(
                     f"""
                     <video autoplay muted controls>
-                        <source src="{url}" type="video/mp4">
+                        <source src="{url}">
                     </video>
                 """,
                     content_type='text/html',