getWriteStream(filePath) {
    const fileObj = this.fileObj;
    const versionName = this.versionName;
    const metadata = { ...fileObj.meta, versionName, fileId: fileObj._id };
    const ret = this.gridFsBucket.openUploadStream(this.fileObj.name, {
      contentType: fileObj.type || 'binary/octet-stream',
      metadata,
    });
    return ret;
  }
returns a write stream
@param filePath if set, use this path
@return the write stream