/**
 * Retry identity for a selected attachment. A file picker returns a fresh File
 * object on reselection; metadata alone can also collide across different bytes.
 */
export async function getMessageFileIdentity(file: Pick<File, "name" | "type" | "arrayBuffer">) {
  const bytes = await file.arrayBuffer()
  const subtle = globalThis.crypto?.subtle
  // Self-hosted HTTP deployments may lack WebCrypto. Exact bytes provide a
  // collision-free in-memory fallback, rather than silently weakening the key.
  const digest = subtle ? await subtle.digest("SHA-256", bytes) : bytes
  const key = Array.from(new Uint8Array(digest), (byte) => byte.toString(16).padStart(2, "0")).join("")
  return JSON.stringify([file.name, file.type, subtle ? "sha256" : "bytes", key])
}