I tried to do a File.Exists and it came back as false.
Nothing fancy, just using:
File.Delete("C:\\Temp\\Cos sie.doc");
These are files with polish filenames. I'm assuming they're using utf8 or
unicode of some kind. File.Delete works ok with filenames of a 'normal'
kind.
How are you specifying the character in the filename? I suggest you use
the unicode escape sequence for the character, ie \uxxxx where xxxx is
the hex number of the character.