You're setting the Content-Type to be multipart/form-data, but then using JSON.stringify on the body data, which returns application/json. You have a content type mismatch.
You will need to encode your data as multipart/form-data instead of json. Usually multipart/form-data is used when uploading files, and is a bit more complicated than application/x-www-form-urlencoded (which is the default for HTML forms).
The specification for multipart/form-data can be found in RFC 1867.
For a guide on how to submit that kind of data via javascript, see here.
The basic idea is to use the FormData object (not supported in IE < 10):
async function sendData(url, data) {
const formData = new FormData();
for(const name in data) {
formData.append(name, data[name]);
}
const response = await fetch(url, {
method: 'POST',
body: formData
});
// ...
}
Per this article make sure not to set the Content-Type header. The browser will set it for you, including the boundary parameter.