Retrieve word document from sharepoint

API design

I’m creating a Word web add-in. As it showed in the image I need to make a POST request from add-in’s web app to some third party API. For this POST request I need to attach the Word document that saved in sharepoint. And I need to attach it with .docx file extention only. How can I use Office.js API Office.context.document.getFileAsync method to handle this scenario. Can I get the Word file with .docx extention from getFileAsync method ?