Levente,
Pharo 1.1 (and 1.1.1) is a bit harder to fix, because there's a bug in MIMEDocument class >> #guessTypeFromName:. This method tries to send #forURIReturnSingleMimeTypeOrDefault: to MIMEType, but that class doesn't implement this method. Actually no class implements this is Pharo 1.1.
But you can avoid this problem by downloading the prebuilt plugin dll from
http://people.inf.elte.hu/uzonyile/ProcessWrapper/2/ProcessWrapperPlugin.dll and copy it to your image or VM folder before loading the package to your image.