Private video clips and wrong thumbnails

I have many private video clips.
In Serviio console I have checked "Generate thumbnails for local videos", but many of my clips has assigned now bad thumbnails (e.g. really movie).
How can I fix this?
Can I somehow remove bad thumbnails?

Sorry, for my 'a rushed query' - I find solution for my troubles here -> https://forum.serviio.org/viewtopic.php?f=4&t=25057&p=120292&hilit=Thumbnail#p120292