Getting error on uploading an asset with mediaType set to either 'application/docx' or 'docx'

when I am trying to upload an asset in step 2 of document generation , I am getting error

{"error":{"code":"Bad Request","message":"The request is missing required parameters, or the values are incorrect."}}. It works perfectly only with 'application/pdf' but I need to upload 'docx' file for template.

How can I upload the word template in the "process of document generation using the word template and json input data for tempalte"?

Thanks,