Support for JSON `Blob`s in formdata

[fire2]

Given the following simplified multipart request:

const object = Objects.assign({}, { name: 'test' });
const formData = new FormData();
formData.append("someJson", new Blob([JSON.stringify(object)], { type: 'application/json' }));
const request = new Request(url, {
  body: formData,
  method: 'POST',
  headers
});

when intercepting this request, formData["someJson"] is just 'blob'. It would be nice to get the object back if the part content type is application/json.