/**
 * Shared attachment utilities for downloading, compressing, and building content blocks.
 * Used by Discord and Slack gateways.
 */
import type { MessageAttachment, ContentBlock } from './types.js';
/**
 * Download a file from URL to local inbound media directory.
 *
 * @param url - File URL to download
 * @param filename - Original filename
 * @param authHeaders - Optional auth headers (e.g., Slack Bearer token)
 * @returns Local file path
 */
export declare function downloadFile(url: string, filename: string, authHeaders?: Record<string, string>): Promise<string>;
/**
 * Compress an image buffer using sharp (progressive JPEG downscaling).
 */
export declare function compressImage(buffer: Buffer, maxSizeBytes: number): Promise<Buffer>;
/**
 * Detect actual image media type from magic bytes.
 */
export declare function detectImageType(buffer: Buffer): string | null;
/**
 * Build content blocks from attachments.
 * Images are base64-encoded for Claude Vision; documents are referenced by path.
 */
export declare function buildContentBlocks(attachments: MessageAttachment[]): Promise<ContentBlock[]>;
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