refactor: prefer Uint8Array & early filename validation

[SukkaW]

The PR introduces two changes:

• Replace Buffer.from with Uint8Array
  • Uint8Array is JavaScript built-in and should be preferred. Buffer is just a subclass of Uint8Array with extra methods we don't need
• Fail early if the finalFilename is invalid, before fetch even begins.

[outloudvi]

Thanks for the change, Sukka!
As there are behavioral changes for invalid filenames, I guess I shall at least bump a minor version.