I added mimetypes to my htaccess file but it didn't affect the .docx file attachments from a K2 article.

How do I add new mimetypes for K2 attachments?

Hello tez,

docx should work fine for k2 attachments. If you need to add more mimetypes you need to edit class.upload.php this file is located in administrator/components/com_k2/lib/

Since docx is already there I'm not sure why it opens as a zip then.

I don't think docx will behave as a document in every browser (like pdf files do).
This is related to the way browsers handle these files, in most browsers (if not all) docx will force download itself.