Assuming that you are performing search using content finder. Go to /libs/wcm/extensions/contentfinder/documents.js and add Mime Type for Docx to "mimeType": ... It is recommended to extend this under /apps if possible.
If global search is failing then it might possible that document is not getting indexed. Check id content search or metadata search is not working and then debug accordingly.
This mimetype is already present in document.js. Anything else that needs to be checked?
Are you using content finder or global search to find your document ?