FormData:
It uses the same format a form would use if the encoding type were set to "multipart/form-data".
https://developer.mozilla.org/en-US/docs/Web/API/FormData
So you are are lying to the server about the encoding by manually specifyingapplication/x-www-form-urlencodedContent-Typeand then giving the.post()method a data structure that will bemultipart/form-data(which is what the boundaries are------WebKitFormBoundaryYuIoxRtV7SIIOnCs)