/**
 * Image format detection from magic bytes.
 *
 * The native Vertex+Anthropic image block needs the correct `mimeType` for
 * each inline image. Buffer and bare-base64 inputs (e.g. Slack / REST uploads)
 * carry no mime hint, so the format must be sniffed from the leading bytes —
 * otherwise a wrong default (historically `image/jpeg`) makes Anthropic reject
 * PNG/GIF/WebP with a media-type mismatch 400.
 */
/**
 * Detect an image's MIME type from its magic bytes. Returns `image/png` for
 * buffers that match no known signature (the safest neutral default for the
 * Vertex image path).
 */
export declare function detectImageMimeType(buffer: Buffer): string;