Or if you are thinking of something more complex you can write a script in perl5, python, or even shell to interact with avconv or ffmpeg to create a thumbnail and then write out an HTML document somewhere in the web server's document root. I don't see avconv in the Raspbian repository so ffmpeg will have to do:
https://networking.ringofsaturn.com/Uni ... mbnail.php
The script would make the thumbnail and fill in the HTML. CSS can be used to make the layout nice. An external CSS file would probably be easiest to work with. Do the CSS last, however.
I have more or less that kind of wrapper as a basic shell script for making thumbnails of JPEG images, but it could be done in python or perl5 as well. The only difference would be how to guess which part of the video to use to get a relevant thumbnail.
Once you have made a wrapper script then you can use incron to trigger it by having incron watch your upload directory. In that way it gets run