[macOS][ChatGPT Desktop] Valid application/zip attachment rejected with HTTP 415 unsupported_mime_type

What version of the Codex App are you using?

• Current ChatGPT Desktop version: 26.814.41407 (build 6720)
• Current bundled Codex CLI: 0.148.0-alpha.15
• Version at reproduction: 26.803.81509

What subscription do you have?

ChatGPT Plus.

What platform is your computer?

macOS 26.5.2 (25F84), Apple Silicon (arm64).

What issue are you seeing?

In ordinary Chat, the desktop file picker accepts a valid ZIP archive, but the upload fails while the application finalizes the file. The local log records:

POST /files/{file_id}/uploaded
HTTP 415
error_code=unsupported_mime_type

The tested archive was independently verified:

• detected MIME type: application/zip;
• size: 4,860,075 bytes;
• archive entries: 192;
• ZIP integrity test: passed.

Three Markdown alternatives from the same project—approximately 455 KB, 49 KB, and 3.6 KB—uploaded successfully. This indicates a ZIP-specific MIME validation or product-surface policy failure rather than a general connection, account, project-content, or file-size problem.

The failure reproduced before and after a complete reset of the local ChatGPT/Codex application profile and a new sign-in.

What steps can reproduce the bug?

1. Create or select a valid ZIP archive with MIME type application/zip.
2. Open an ordinary Chat in ChatGPT Desktop on macOS.
3. Add the ZIP using the attachment picker.
4. Wait for the application to complete the upload.
5. The attachment displays an upload failure.
6. The desktop log shows HTTP 415 unsupported_mime_type for POST /files/{file_id}/uploaded.

The problem reproduced at:

• 2026-08-13 22:56:34 CEST;
• 2026-08-14 13:06:10 CEST, after the profile reset;
• 2026-08-14 13:22:36 CEST, after the profile reset.

What is the expected behavior?

If ZIP attachments are supported in ordinary Chat, a valid application/zip archive should upload successfully.

If ZIP attachments are intentionally unsupported for this surface or account, the file picker should reject the file before starting the upload and display a clear explanation. The application should not accept the selection and then fail at the finalization endpoint with a generic upload error.

Additional information

Related feedback session IDs accepted by the application:

• no-active-thread-019ffcea-d514-79e1-8ee2-b46546217420
• no-active-thread-01a00002-d02e-77e1-9788-06f9270c9395

The full local profile reset completed at 2026-08-14 13:03:54 CEST. The same HTTP 415 failure occurred after the reset, which makes stale local cache or profile corruption an unlikely explanation.

A search of existing openai/codex issues on August 18, 2026 did not find a report matching ZIP upload rejection with unsupported_mime_type.

No archive contents, local paths, account data, conversation content, or full logs are included. The archive itself can be provided privately if a maintainer requests a reproduction sample.