Better files filtering in Dokeos 1.8.5

It's already in the code for Dokeos 1.8.5 since 2 weeks ago: files extensions filtering. It was possible, in 1.8.4, to filter files extensions coming from ZIP files, but I didn't have time back then to include a widespread file filtering. It is included now. There is still a problem whereby Windows interprets files looking inside them, so it does not need the extension to be wrong in order to execute the file as is needed for a virus to spread.