DOpus incorrectly displays polish letters in thumbnails when using UTF-8.
If I switch to Ansi-1250, everything is OK.
You need to save the file with a BOM, otherwise Opus will not know it's UTF-8.
I just found in Text-File Thumbnails plugin configuration an option where (I think) I can choose default encoding (for each extension separately).
Thank you, anyway, for another solution.