Contractor contract doesn't work in pantheon-files

Question

I regularly need to extract ZIP archives that contain characters that are encoded in Chinese (GB18030). The slow way of doing this is to open the folder in a terminal and entering the following command:

$ unzip -O GB18030 foo.zip

I want to create a contractor contract that will allow me to unzip files from the right-click menu in pantheon-files. I placed the following contract (unzip-chinese.contract) in /usr/share/contractor/

[Contractor Entry]
Name=Extract GB18030
Icon=extract-archive
Description=Extract the GB18030 encoded archive and quit
MimeType=application/zip;
Exec=unzip -O GB18030 %f

Can anybody tell me why when I select this option from the right-click menu, nothing happens?

Thanks!