refactor: message content file handling [BC BREAK]

[chr-hertel]

Preparation for #279

• Introducing a base File that centralizes mime type, base64 handling, etc.
• Image split into ImageUrl and Image based on File => i usually know what i need in user land
  • new Image('https://...') => new ImageUrl('https://...)
  • new Image('data:image/jpeg;base64,...') => Image::fromDataUrl('data:image/jpeg;base64,...')
  • new Image('/path/to/jpeg.jpg') => Image::fromFile('/path/to/jpeg.jpg')
• removing File class of Whisper bridge in favor of Audio