The preview code will not work without an extension... i just tested the new code and that will not work either, we require file extensions to determine some javascript functionality, feed files to players etc.. not so much in the new codes... but the old code was even more reliant on the extension. Both ToG and Terayon will have this loss of functionality without extensions.
The thumbnails aren't visible because the file extension is essential to the name of the file of the previews.
Ino if i were a user of a website i'd want to know the extension of a file before i download it anyway.
So leave that option off, and you'll have no problems. I'll put this in the readme when we release.