Function detectImageFormatFromBuffer

src/utils/imageResizer.ts:769–812

(buffer: Buffer)

 * @returns Media type string (e.g., 'image/png', 'image/jpeg') or 'image/png' as default
 */
export function detectImageFormatFromBuffer(buffer: Buffer): ImageMediaType {
  if (buffer.length < 4) return 'image/png' // default

  // Check PNG signature
  if (
    buffer[0] === 0x89 &&
    buffer[1] === 0x50 &&
    buffer[2] === 0x4e &&
    buffer[3] === 0x47
  ) {
    return 'image/png'
  }

  // Check JPEG signature (FFD8FF)
  if (buffer[0] === 0xff && buffer[1] === 0xd8 && buffer[2] === 0xff) {
    return 'image/jpeg'
  }

  // Check GIF signature (GIF87a or GIF89a)
  if (buffer[0] === 0x47 && buffer[1] === 0x49 && buffer[2] === 0x46) {
    return 'image/gif'
  }

  // Check WebP signature (RIFF....WEBP)
  if (
    buffer[0] === 0x52 &&
    buffer[1] === 0x49 &&
    buffer[2] === 0x46 &&
    buffer[3] === 0x46
  ) {
    if (
      buffer.length >= 12 &&
      buffer[8] === 0x57 &&
      buffer[9] === 0x45 &&
      buffer[10] === 0x42 &&
      buffer[11] === 0x50
    ) {
      return 'image/webp'
    }
  }

  // Default to PNG if unknown
  return 'image/png'
}

/**
 * Detect image format from base64 data using magic bytes