public sendFile(file: File, visibility: 'public' | 'private' = 'private'): Promise<SocrateFile> {
    const formData = new FormData();
    const xhr = new XMLHttpRequest();
    const url = `${environment.API_URL}files?visibility=${visibility}`;

    formData.append('file', file, file.name);
    return this.auth.getJWT().then(jwt => {
      xhr.open('POST', url, true);
      xhr.setRequestHeader('Authorization', jwt);
      return new Promise<SocrateFile>((res, rej) => {
        xhr.onload = () => {
          const response = JSON.parse(xhr.response);

          if (!response.id) {
            console.error(response);
            rej();
          } else {
            res(response);
          }
        };
        xhr.send(formData);  // multipart/form-data
      });
    });
  }