Hi @PeteBoyd out of curiosity was the .docx file generated by Microsoft word natively or did you save a file to .docx from something like Libre office?
A quick google search returns the below:
https://community.adobe.com/t5/coldfusion/mime-type-issue-docx/m-p/1775235?page=1
https://stackoverflow.com/questions/1201945/how-is-mime-type-of-an-uploaded-file-determined-by-browser