lib/attachment-preview.ts

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import { normalizeMimeType } from "@/lib/inline-safe";

export type AttachmentPreviewKind = "image" | "document" | "audio" | "video";

/** Add a presentation preference without changing signed attachment parameters. */
export function withAttachmentPreviewTheme(src: string, theme: "light" | "dark"): string {
  const hashIndex = src.indexOf("#");
  const hash = hashIndex === -1 ? "" : src.slice(hashIndex);
  const base = hashIndex === -1 ? src : src.slice(0, hashIndex);
  const queryIndex = base.indexOf("?");
  const path = queryIndex === -1 ? base : base.slice(0, queryIndex);
  const params = new URLSearchParams(queryIndex === -1 ? "" : base.slice(queryIndex + 1));
  params.set("theme", theme);
  return `${path}?${params}${hash}`;
}

const CSV_MIME_TYPES: ReadonlySet<string> = new Set([
  "text/csv",
  "text/x-csv",
  "application/csv",
  "application/x-csv",
]);

/** Recognize CSV exports even when storage reports a generic or Excel MIME type. */
export function isCsvAttachment(
  rawMimeType: string | null | undefined,
  fileName?: string | null,
): boolean {
  return CSV_MIME_TYPES.has(normalizeMimeType(rawMimeType)) ||
    (typeof fileName === "string" && /\.csv$/i.test(fileName.trim()));
}

/** Identify Markdown attachments by their user-visible filename. */
export function isMarkdownFileName(
  fileName: string | null | undefined,
): boolean {
  return typeof fileName === "string" && /\.md$/i.test(fileName.trim());
}

/** Plain text filename fallback for uploads with generic MIME metadata. */
export function isPlainTextAttachment(rawMimeType: string | null | undefined, fileName?: string | null): boolean {
  return normalizeMimeType(rawMimeType) === "text/plain" ||
    (typeof fileName === "string" && /\.(txt|text)$/i.test(fileName.trim()));
}

const DOCUMENT_MIME_TYPES: ReadonlySet<string> = new Set([
  "application/javascript",
  "application/json",
  "application/pdf",
  "application/toml",
  "application/xml",
  "application/x-yaml",
  "application/xhtml+xml",
  "application/yaml",
]);

/**
 * Return the browser surface that can display an attachment, or null when the
 * file should remain download-only. Active documents are rendered by the
 * sandboxed preview endpoint and iframe, never by the app document itself.
 */
export function attachmentPreviewKind(
  rawMimeType: string | null | undefined,
  fileName?: string | null,
): AttachmentPreviewKind | null {
  const mimeType = normalizeMimeType(rawMimeType);
  if (isMarkdownFileName(fileName) || isCsvAttachment(mimeType, fileName) || isPlainTextAttachment(mimeType, fileName)) {
    return "document";
  }
  if (!mimeType) return null;

  if (mimeType.startsWith("audio/")) return "audio";
  if (mimeType.startsWith("video/")) return "video";
  if (mimeType.startsWith("image/") && mimeType !== "image/svg+xml") {
    return "image";
  }
  if (
    mimeType.startsWith("text/") ||
    DOCUMENT_MIME_TYPES.has(mimeType) ||
    mimeType.endsWith("+json") ||
    mimeType.endsWith("+xml")
  ) {
    return "document";
  }
  return null;
}