Better control for thumbnail

[naglalakk]

In upload we generate a thumbnail. This doesn't handle video formats in any way and becomes very heavy and bloated when we drop media files in there. Maybe we can find a way to provide a snapshot thumbnail of videos. We also have to handle files that don't really have thumbnails like text files etc. For this I think a config option would be helpful where we can specify the path of the "default" thumbnail image