[Bug]: Pasting an unsupported document leaves the chat unrecoverable

Before submitting

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

Area

Not sure

Summary

Pasting an unsupported document into a T3 Code chat produces a media-type error and leaves the conversation unusable. There is no apparent way to recover and continue the same chat.

Steps to reproduce

Reported sequence:

1. Open a chat in T3 Code.
2. Paste a document that is not supported. In this case, the error identifies the file part as text/html.
3. Try to continue the conversation after the error appears.

The original document and a clean minimal reproduction are not available in this report.

Expected behavior

Reject unsupported attachments with an actionable error before they leave the conversation in a failed state. If an attachment fails after submission, provide a way to remove or discard it and continue the existing chat.

Actual behavior

The chat shows:

'file part media type text/html' functionality not supported.

After this error, the user cannot recover and continue the same conversation. The problem is the loss of a usable chat after an unsupported attachment is accepted.

Impact

Blocks work completely in the affected chat.

Version or commit

Not provided. Observed on September 16, 2026.

Environment

The affected provider/model and exact app version have not been confirmed.

Screenshots, recordings, or supporting files

The reporter supplied a screenshot showing the error above in red. The error is transcribed verbatim here.

Workaround

No recovery workaround was reported.

Related issues

• #11295 reports a similar unrecoverable-session outcome after browser screenshot tool results are stored in provider history. This report concerns a pasted document rejected as text/html; a shared root cause has not been established.
• #7225 covered unsupported HEIC attachments failing a turn and was closed after attach-time validation was added.

No files or tests are named in the report. Reproduce the paste of a text/html document, then trace attachment validation and the chat error state to determine why the conversation cannot continue. Done means unsupported attachments are rejected with an actionable error, or can be removed so the existing chat remains usable.