/**
 * Synchronously reads up to `maxBytes` from the start of `filePath` and returns `op(header)`.
 * If the file is shorter, `header` is only the bytes actually read.
 */
export declare function peekFileSync<T>(filePath: string, maxBytes: number, op: (header: Uint8Array) => T): T;
/**
 * Like {@link peekFileSync} but uses async I/O.
 */
export declare function peekFile<T>(filePath: string, maxBytes: number, op: (header: Uint8Array) => T): Promise<T>;