[Bug]: Mobile directory links open a file preview and show File unavailable #21257

Before submitting

• I searched existing issues and did not find a duplicate.
• I included enough detail to reproduce or investigate the problem.

Area

apps/mobile

Steps to reproduce

1. Pair the native mobile app with a development server and open a conversation whose workspace is /tmp/t3-mobile-folder-links/workspace.
2. Create an existing sibling directory /tmp/t3-mobile-folder-links/shots containing a text file or screenshot.
3. Have an assistant message contain [shots](/tmp/t3-mobile-folder-links/shots).
4. Tap the directory link in the mobile conversation.

A directory named images.png also needs to be treated as a directory when linked from chat, even though its name looks like an image file.

Expected behavior

Open the existing file browser rooted at the linked directory. Selecting a child should open that file; Back should return to the directory and then the conversation. Ordinary image-file links should retain their native preview.

Actual behavior

Mobile routes the directory to a file preview and shows File unavailable:

Failed to read workspace file '/tmp/t3-mobile-folder-links/shots' in '/tmp/t3-mobile-folder-links/workspace'.

The directory exists. The server correctly rejects reading it as a file (path_not_file); mobile needs to distinguish a directory from a file before choosing the preview route. The web client already has a directory-browser path for this case.

Impact

Minor bug or occasional failure

Version or commit

Reproduced during the investigation on upstream commit 16918b57d6.

Environment

Native Android development client on Pixel 10 emulator / Android 17, paired with an isolated local development backend. Native reproduction and before/after evidence are recorded in #21073.

Screenshots, recordings, or supporting files

The proposed fix and after screenshots/recording are in #21073.

Related reports

• [Bug]: A chat link to a folder outside the workspace opens a failing file preview #15215 describes the web/desktop version and is already closed. This report tracks the mobile directory-routing failure.
• [Bug]: Mobile file links outside the workspace silently do nothing #7798 concerned mobile external-file links silently doing nothing and is also closed. Here navigation occurs, but a directory enters the file-only preview.

Workaround

Link directly to a known child file instead of its containing directory.