Hi, the file needs to be in Blob format as it's defined in the specification. I'll recommend you to separate the object and file in different keys inside the FormData. Anyways to avoid the [object, object] it's because you need to serialize the object before append to the form data like this:
formData.append('my-object', JSON.stringify(obj));
https://developer.mozilla.org/en-US/docs/Web/API/FormData/Using_FormData_Objects https://developer.mozilla.org/en-US/docs/Web/API/FormData/append