If your browser is capable Videos will play in the browser from the folder and Media applications, and from embedded in Pages etc;

<video width="320" height="240" controls>
    <source src="/ShowItemData.po?path=/folder/example.mp4" type="video/mp4">
    <source src="/ShowItemData.po?handle=1&filename=example.ogg" type="video/ogg">
    <source src="//domain.tld/example.webm" type="video/webm">
    Please use a modern browser for video support.
</video>

Tools:

• ffmpeg can be used to thumbnail and transcode.
  • Noodle will thumbnail images and videos for you.
  • make sure to encode with the "faststart" flag;

    ffmpeg -i in.mp4 -codec copy -movflags +faststart out.mp4

• MediaInfo can be used to inspect.
  • Verify faststart/IsStreamable;

    mediainfo -f file.mp4 | grep IsStreamable

• handbrake is a nice GUI for encoding
  • make sure to encode with "web optimized"
• djpeg + pngtopnm + pnmscale can be used to thumbnail non-progressive jpgs of any size with minimal RAM usage.