Videos can be embedded in HTML pages

<video width="320" height="240" controls="controls" autoplay="autoplay">
  <source src="movie.mp4" type="video/mp4">
Your browser does not support the video tag.
</video>

404.3

When IIS delivers a mail message to a mail application, or a Web page to a client Web browser, it also sends the MIME type of the data it is sending. If there is an attached or embedded file in a specific format, IIS tells the client application the MIME type of the embedded or attached file. The client application then knows how to process or display the data being received from IIS. IIS returns error 404.3 if a client request refers to a file name extension that is not defined in the MIME types.

Add MIME type

1) Select the site to configure in IIS, right click and select "Properties"
2) Under HTTP Headers Tab, select "File Types" under the MIME Map section and select "New Type"
3) Type ".flv" as the associated extension and "video/x-flv" as the content type or "flv-application/octet-stream" I'm not sure on which one gives here.
4) for .mp4 files type ".mp4" as the extension and "video/mp4" as the mime type (this one I tested personally)
4) Select "OK",
5) type services.msc, find the "World Wide Web Publishing Service" and click on the restart icon on top or open up and choose restart