Paperclip 4.x includes automatic file content type detection:
If you want to include detection of newer Microsoft Office and OpenOffice XML formatted documents you may need to upgrade the
file utility which Paperclip runs. My version of OSX (Mountain Lion) was running 5.04. If you don't update the office documents will be detected as zip files.
The latest version can be found at:
tar -xzvf file-5.19.tar.gz
sudo make install