I tried using it that way, but the same result: PDF files are being opened but PNG files do nothing. I am not getting any errors or anything, it just doesn't do anything.
Edit: I forgot catching the CLR error. Now i get the same error as i do when using WinApi::ShellExecute. No application is associated with the specified file for this operation. But as i said before, the default for PNG has been set so i don't understand the error.