Your Word documents are being saved with the mimetype "application/octet-stream" rather than "application/msword". This can happen if you save directly to the Alfresco CIFS interface (see http://issues.alfresco.com/browse/AR-1636 for details)
This has been fixed in the 2.1 Enterprise branch and will be part of the community update released in a few weeks.
The workaround is to either save locally then copy to the CIFS path, or use the web client to upload.