constructor(
    length,
    initialData,
    progressiveDone = false,
    contentDispositionFilename = null
  ) {
    this.length = length;
    this.initialData = initialData;
    this.progressiveDone = progressiveDone;
    this.contentDispositionFilename = contentDispositionFilename;
  }
@param {number} length
@param {Uint8Array|null} initialData
@param {boolean} [progressiveDone]
@param {string} [contentDispositionFilename]