fix(mobile): align !file card preprocess with web parser + CLI escaped labels (MUL-4287)

Howard final-review blocker: mobile's `!file[...]` preprocess didn't keep up
with the CLI's file-card output, so agent-produced non-image files rendered
nowhere on mobile.

- `FILE_LINE_RE` used `[^\]]+` for the label, so the CLI's escaped-bracket
  output `!file[a\]b.pdf](url)` (cmd_attachment.go escapeMarkdownLabel) never
  matched — the line stayed literal AND `standaloneAttachments` still hid the
  fallback card (the URL is in `content`), so the file showed nowhere.
- Align the matcher with web's `packages/ui/markdown/file-cards.ts`: label
  allows backslash-escaped metacharacters (ReDoS-safe class), and the URL is
  restricted to the same allowlist (site-relative /uploads + /api/attachments/
  <UUID>/download, plus absolute http(s)); disallowed schemes stay plain text.
- Unescape the label to the real filename, then re-escape only the chars that
  would break a markdown LINK label (mobile emits `[📎 name](url)`, re-parsed
  by the renderer — unlike web's HTML data-filename), so a raw `]` never
  truncates the link text.

No dedup change: once the inline `!file` renders, hiding the standalone card is
correct. Added focused unit tests covering the escaped-label case, parens/
backslash unescape, the site-relative URL form, and disallowed-scheme rejection.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Co-authored-by: multica-agent <github@multica.ai>

apps/mobile/lib/markdown/preprocess.test.ts

@@ -0,0 +1,55 @@
+import { describe, expect, it } from "vitest";
+import { preprocessMobileMarkdown } from "./preprocess";
+
+const UUID = "019f49e2-5b07-7970-beef-c0d537fb8c1d";
+const ABS_URL = `https://multica-app.copilothub.ai/api/attachments/${UUID}/download`;
+const REL_URL = `/api/attachments/${UUID}/download`;
+
+describe("preprocessMobileMarkdown — !file file cards", () => {
+  it("matches the CLI's escaped-bracket label and keeps it markdown-safe", () => {
+    // CLI emits `a]b.pdf` escaped as `a\]b.pdf` (cmd_attachment.go
+    // escapeMarkdownLabel). The old regex stopped at the first `]` and left the
+    // line literal; now it is captured whole and re-emitted as a tappable link
+    // whose label stays escaped so the `]` doesn't truncate the link text.
+    const out = preprocessMobileMarkdown(`!file[a\\]b.pdf](${ABS_URL})`);
+    expect(out).toBe(`[📎 a\\]b.pdf](${ABS_URL})`);
+  });
+
+  it("unescapes non-breaking metacharacters (parens) in the displayed name", () => {
+    // Parens are legal inside a markdown link label, so they are unescaped for
+    // display and not re-escaped.
+    const out = preprocessMobileMarkdown(`!file[report\\(1\\).pdf](${ABS_URL})`);
+    expect(out).toBe(`[📎 report(1).pdf](${ABS_URL})`);
+  });
+
+  it("unescapes an escaped backslash in the label", () => {
+    const out = preprocessMobileMarkdown(`!file[a\\\\b.pdf](${ABS_URL})`);
+    expect(out).toBe(`[📎 a\\\\b.pdf](${ABS_URL})`);
+  });
+
+  it("renders a plain (unescaped) label", () => {
+    const out = preprocessMobileMarkdown(`!file[notes.txt](${ABS_URL})`);
+    expect(out).toBe(`[📎 notes.txt](${ABS_URL})`);
+  });
+
+  it("accepts the site-relative /api/attachments URL form (web parity)", () => {
+    const out = preprocessMobileMarkdown(`!file[a.pdf](${REL_URL})`);
+    expect(out).toBe(`[📎 a.pdf](${REL_URL})`);
+  });
+
+  it("leaves a disallowed-scheme URL as plain text (no out-of-band navigation)", () => {
+    const line = `!file[x.pdf](javascript:alert(1))`;
+    expect(preprocessMobileMarkdown(line)).toBe(line);
+  });
+
+  it("does not touch inline images (![...] is not a file card)", () => {
+    const line = `![chart.png](${ABS_URL})`;
+    expect(preprocessMobileMarkdown(line)).toBe(line);
+  });
+
+  it("only transforms the standalone file-card line, leaving surrounding text", () => {
+    const input = `here is the file\n\n!file[a\\]b.pdf](${ABS_URL})\n\nlet me know`;
+    const output = `here is the file\n\n[📎 a\\]b.pdf](${ABS_URL})\n\nlet me know`;
+    expect(preprocessMobileMarkdown(input)).toBe(output);
+  });
+});

apps/mobile/lib/markdown/preprocess.ts

@@ -23,7 +23,39 @@
  */
 import { preprocessMentionShortcodes } from "@multica/core/markdown";
 
-const FILE_LINE_RE = /^!file\[([^\]]+)\]\((https?:\/\/[^)]+)\)$/;
+// File-card line matcher, kept in sync with web's parser in
+// `packages/ui/markdown/file-cards.ts` (NEW_FILE_CARD_RE + FILE_CARD_URL_PATTERN):
+//
+//   - Label allows backslash-escaped metacharacters (`\[ \] \\ \( \)`) so a
+//     filename like `a]b.pdf` — which the CLI escapes to `a\]b.pdf` in its
+//     `!file[...]` output (see cmd_attachment.go escapeMarkdownLabel) — is
+//     captured whole. Backslash is excluded from the negated class so
+//     overlapping alternatives can't backtrack (ReDoS, web #4881).
+//   - URL is restricted to the same allowlist web accepts: site-relative
+//     `/uploads/...` and `/api/attachments/<UUID>/download`, plus absolute
+//     `http(s)://`. Anything else (`javascript:`, `data:`, `//host`, other
+//     `/api/…`) is left as plain text so a stored card can't become an
+//     out-of-band navigation.
+const ATTACHMENT_UUID =
+  "[0-9a-fA-F]{8}-[0-9a-fA-F]{4}-[0-9a-fA-F]{4}-[0-9a-fA-F]{4}-[0-9a-fA-F]{12}";
+const FILE_CARD_URL = `/uploads/[^)]*|https?://[^)]+|/api/attachments/${ATTACHMENT_UUID}/download`;
+const FILE_LINE_RE = new RegExp(
+  `^!file\\[((?:\\\\.|[^\\]\\\\])*)\\]\\((${FILE_CARD_URL})\\)$`,
+);
+
+// Unescape the file-card label back to the real filename (mirrors web's
+// `newMatch[1].replace(/\\([[\]\\()])/g, "$1")`).
+function unescapeFileLabel(label: string): string {
+  return label.replace(/\\([[\]\\()])/g, "$1");
+}
+
+// Re-escape only the characters that would break a markdown LINK label, so the
+// emitted `[📎 name](url)` stays valid markdown. Mobile's target is a link
+// (re-parsed by marked / the enriched renderer), unlike web's HTML
+// `data-filename` attribute — so a raw `]` must not truncate the link text.
+function escapeLinkLabel(name: string): string {
+  return name.replace(/([\\[\]])/g, "\\$1");
+}
 
 function preprocessFileCards(input: string): string {
   return input
@@ -31,7 +63,8 @@ function preprocessFileCards(input: string): string {
     .map((line) => {
       const m = line.trim().match(FILE_LINE_RE);
       if (!m) return line;
-      return `[📎 ${m[1]}](${m[2]})`;
+      const label = escapeLinkLabel(unescapeFileLabel(m[1]!));
+      return `[📎 ${label}](${m[2]})`;
     })
     .join("\n");
 }