Hello,
UPDATE: phpdocx 14.5 reordered the default base template to return Microsoft Word 2007+ mime type using CreateDocx.
A DOCX file is a ZIP file, so application/octet-stream is a correct MIME for a DOCX file; ZipArchive, used to create the DOCX files, generates that mime. If you want to set other MIME you can use header functions from PHP when sending/downloading the documents (as createDocxAndDownload does) or you can try changing the magic setup in your server:
https://stackoverflow.com/questions/6595183/docx-file-type-in-php-finfo-file-is-application-zip
As you are using a Premium license, you can also use the stream mode (https://www.phpdocx.com/api-documentation/performance/zip-stream-docx-with-PHP).
If you are using an external service that requires application/vnd.openxmlformats-officedocument.wordprocessingml.document as mime type, we recommend you to contact them to know how to set other mime type to their service (as PHP header function allows).
Regards.