In short, you can't specify custom mime types using Google AppEngine e-mail.
In order to send e-mail via Google AppEngine through Google you need to use the Mail API provided. You can see why .docx is using the content type application/msword
here: https://cloud.google.com/appengine/docs/standard/python/mail/mail-with-headers-attachments. You might want to submit an issue to the issue tracker in order to resolve this.
It may be worth looking at 3rd party e-mail providers such as Amazon's SES or SendGrid. I've personally been using Amazon's SES to get around a lot of the lack of features and restrictions in GAE's Mail API (though Google is constantly improving this API).
You can use the boto library to communicate with AWS SES and send RAW e-mail messages. (Look at this question for an example on how to create the e-mail, small modifications may be needed for your purposes)