fix(android): open attachments directly with default viewer app

src/lib/__tests__/email-export.test.ts

@@ -0,0 +1,144 @@
+import { describe, it, expect, vi, beforeEach } from 'vitest';
+
+// Platform.OS is flipped per test, so hoist a mutable object into the module
+// mock rather than using the fixed 'android' stub from test-setup.
+const { platform } = vi.hoisted(() => ({ platform: { OS: 'android' as string, Version: 33 } }));
+
+vi.mock('react-native', () => ({
+  Platform: platform,
+  NativeModules: {},
+  NativeEventEmitter: class {
+    addListener() {
+      return { remove: () => undefined };
+    }
+  },
+}));
+
+// A `File` that records both URL forms, so tests can assert which one each
+// handoff path was given: expo-sharing needs `file://`, an Intent needs
+// `content://`.
+vi.mock('expo-file-system', () => {
+  class File {
+    name: string;
+    uri: string;
+    contentUri: string;
+    exists = true;
+    constructor(dir: { uri: string }, name: string) {
+      this.name = name;
+      this.uri = `${dir.uri}${name}`;
+      this.contentUri = `content://org.bulwarkmail.mobile.FileSystemFileProvider/${name}`;
+    }
+    create() {}
+    delete() {}
+    write() {}
+    static downloadFileAsync = vi.fn(async () => undefined);
+  }
+  return {
+    File,
+    Directory: class {},
+    Paths: { cache: { uri: 'file:///cache/' }, document: { uri: 'file:///documents/' } },
+  };
+});
+
+vi.mock('expo-intent-launcher', () => ({
+  startActivityAsync: vi.fn(async () => ({ resultCode: -1 })),
+}));
+
+vi.mock('expo-sharing', () => ({
+  isAvailableAsync: vi.fn(async () => true),
+  shareAsync: vi.fn(async () => undefined),
+}));
+
+vi.mock('../client-cert', () => ({
+  getClientCertAlias: vi.fn(async () => null),
+  secureFetch: vi.fn(),
+}));
+
+vi.mock('../../api/jmap-client', () => ({
+  jmapClient: { authHeader: 'Bearer token' },
+}));
+
+vi.mock('../../api/blob', () => ({
+  getDownloadUrl: vi.fn((blobId: string, name: string) => `https://mail.example.com/${blobId}/${name}`),
+}));
+
+import * as IntentLauncher from 'expo-intent-launcher';
+import * as Sharing from 'expo-sharing';
+import { shareAttachment, downloadAttachment } from '../email-export';
+
+const VIEW = 'android.intent.action.VIEW';
+const FLAG_GRANT_READ_URI_PERMISSION = 0x00000001;
+
+describe('shareAttachment (preview)', () => {
+  beforeEach(() => {
+    platform.OS = 'android';
+    vi.mocked(IntentLauncher.startActivityAsync).mockClear().mockResolvedValue({ resultCode: -1 });
+    vi.mocked(Sharing.shareAsync).mockClear();
+  });
+
+  it('hands the file to a viewer app via a VIEW intent on Android', async () => {
+    await shareAttachment('blob-1', 'report.pdf', 'application/pdf');
+
+    expect(IntentLauncher.startActivityAsync).toHaveBeenCalledWith(VIEW, {
+      data: 'content://org.bulwarkmail.mobile.FileSystemFileProvider/report.pdf',
+      type: 'application/pdf',
+      flags: FLAG_GRANT_READ_URI_PERMISSION,
+    });
+    // The share sheet is what the direct handoff exists to avoid.
+    expect(Sharing.shareAsync).not.toHaveBeenCalled();
+  });
+
+  it('falls back to the share sheet when no app handles the type', async () => {
+    vi.mocked(IntentLauncher.startActivityAsync).mockRejectedValue(
+      new Error('No Activity found to handle Intent'),
+    );
+
+    await shareAttachment('blob-2', 'weird.xyz', 'application/x-weird');
+
+    // file:// — expo-sharing rejects content:// URLs outright.
+    expect(Sharing.shareAsync).toHaveBeenCalledWith('file:///cache/weird.xyz', {
+      mimeType: 'application/x-weird',
+      dialogTitle: 'weird.xyz',
+    });
+  });
+
+  it('surfaces a download failure instead of silently falling back', async () => {
+    const { File } = await import('expo-file-system');
+    vi.mocked(File.downloadFileAsync).mockRejectedValueOnce(new Error('Download failed: 404'));
+
+    await expect(shareAttachment('blob-3', 'gone.pdf', 'application/pdf')).rejects.toThrow(
+      'Download failed: 404',
+    );
+    expect(IntentLauncher.startActivityAsync).not.toHaveBeenCalled();
+  });
+
+  it('uses the share sheet on iOS', async () => {
+    platform.OS = 'ios';
+
+    await shareAttachment('blob-4', 'photo.jpg', 'image/jpeg');
+
+    expect(IntentLauncher.startActivityAsync).not.toHaveBeenCalled();
+    expect(Sharing.shareAsync).toHaveBeenCalledWith('file:///cache/photo.jpg', {
+      mimeType: 'image/jpeg',
+      dialogTitle: 'photo.jpg',
+    });
+  });
+});
+
+describe('downloadAttachment (save a copy)', () => {
+  beforeEach(() => {
+    platform.OS = 'android';
+    vi.mocked(IntentLauncher.startActivityAsync).mockClear().mockResolvedValue({ resultCode: -1 });
+    vi.mocked(Sharing.shareAsync).mockClear();
+  });
+
+  it('keeps the share sheet on Android so the save dialog stays reachable', async () => {
+    await downloadAttachment('blob-5', 'report.pdf', 'application/pdf');
+
+    expect(IntentLauncher.startActivityAsync).not.toHaveBeenCalled();
+    expect(Sharing.shareAsync).toHaveBeenCalledWith('file:///documents/report.pdf', {
+      mimeType: 'application/pdf',
+      dialogTitle: 'report.pdf',
+    });
+  });
+});

src/lib/email-export.ts

@@ -1,4 +1,6 @@
+import { Platform } from 'react-native';
 import { Directory, File, Paths } from 'expo-file-system';
+import * as IntentLauncher from 'expo-intent-launcher';
 import * as Sharing from 'expo-sharing';
 import { jmapClient } from '../api/jmap-client';
 import { getDownloadUrl } from '../api/blob';
@@ -12,6 +14,7 @@ import {
 import { getClientCertAlias, secureFetch } from './client-cert';
 
 const RFC822 = 'message/rfc822';
+const FLAG_GRANT_READ_URI_PERMISSION = 0x00000001;
 
 // Read the user's filename template + transform preferences for exports.
 function emailFileOptions(): EmailFilenameOptions {
@@ -48,7 +51,32 @@ function safeAttachmentName(name: string | undefined, type: string | undefined):
 // expo-sharing only accepts `file://` URLs and rejects `content://` with
 // "Only local file URLs are supported". On Android it then wraps the file
 // itself with its bundled SharingFileProvider before launching the share
-// intent, so we must NOT pre-translate to `file.contentUri`.
+// intent, so every `Sharing.shareAsync` below gets `downloaded.uri` and must
+// NOT be pre-translated to `downloaded.contentUri`. `openWithViewer` is the
+// one exception: it assembles the intent itself, so there it's the reverse —
+// only a content URI is grantable to another app.
+
+// Android-only: hand the file to whichever app owns the type (PDF viewer,
+// gallery, video player, ...) rather than to the share sheet. Returns false
+// when the handoff didn't happen so the caller can fall back to sharing —
+// most often because no installed app handles the MIME type, in which case
+// startActivityAsync rejects with ActivityNotFoundException.
+async function openWithViewer(file: File, mimeType: string): Promise<boolean> {
+  try {
+    await IntentLauncher.startActivityAsync('android.intent.action.VIEW', {
+      // Read-only grant, scoped to the receiving app for the life of the
+      // intent. Deliberately no FLAG_ACTIVITY_NEW_TASK: expo-intent-launcher
+      // uses startActivityForResult, and a new task cancels that result.
+      data: file.contentUri,
+      type: mimeType,
+      flags: FLAG_GRANT_READ_URI_PERMISSION,
+    });
+    return true;
+  } catch (e) {
+    console.warn('[attachments] no viewer for', mimeType, '- falling back to share:', e);
+    return false;
+  }
+}
 
 // Routes the download via the client-cert-aware native module when the user
 // has picked a cert, and via expo-file-system's native streaming downloader
@@ -99,6 +127,10 @@ export async function shareAttachment(
   const dest = new File(Paths.cache, filename);
   const url = getDownloadUrl(blobId, filename, mimeType, accountId);
   const downloaded = await downloadInto(url, dest, Paths.cache);
+
+  if (Platform.OS === 'android' && (await openWithViewer(downloaded, mimeType))) {
+    return;
+  }
   if (!(await Sharing.isAvailableAsync())) {
     throw new Error('Sharing is not available on this device');
   }
@@ -111,8 +143,9 @@ export async function shareAttachment(
 // Save-to-disk variant. iOS doesn't expose a user-visible "Downloads" folder,
 // so on both platforms we land the file in the document directory and hand it
 // to the share sheet — which on iOS surfaces "Save to Files" and on Android
-// surfaces the system save dialog. The 'preview' counterpart (shareAttachment)
-// uses cache + share, which on most viewers opens directly without prompting.
+// surfaces the system save dialog. Unlike the 'preview' counterpart
+// (shareAttachment), this one keeps the share sheet on Android as well: "save
+// a copy" is a share-sheet destination, not something a viewer app handles.
 export async function downloadAttachment(
   blobId: string,
   name: string | undefined,