fix(session): read text attachments as text for non-text/plain MIME types

[adityachaudhary99]

Issue for this PR

Closes #17301

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Attaching a text file could send binary-looking garbage to the model instead of the file's contents. resolveUserPart (session/prompt.ts) only routed an attachment through the readable-text path (the Read tool) when its MIME was exactly text/plain. A file with no extension — or any unrecognized text file — resolves to application/octet-stream, so it skipped that path and hit the branch that base64-encodes the raw bytes and prepends a synthetic Called the Read tool… frame. The model then received a data-URL of raw bytes, which is the garbled output in the issue.

The earlier #26152 fixed the run --file producer (real MIME lookup), but the consumer here still only recognized literal text/plain, so the interactive/upload path stayed broken (and extension-less files arguably got worse, since they now resolve to octet-stream).

The fix gates the text path on a shared isTextMime() check (text/*, JSON/XML/JS, empty, and the application/octet-stream unknown fallback) instead of an exact string. The Read tool already content-sniffs, so real text reads cleanly, sniffed images/PDFs still become proper attachments, and genuine binary now fails closed with an honest message instead of a garbled blob. Image/PDF MIMEs are untouched.

How did you verify your code works?

Added two cases to test/session/prompt.test.ts: a text file attached as application/octet-stream (the exact repro — an extension-less file) and as text/markdown both produce a clean synthetic text part containing the file's text, with zero forwarded binary parts. I couldn't run the full suite in my isolated worktree (the @opencode-ai/core workspace alias resolves to a stale checkout there), so I rely on CI for the suite.

Screenshots / recordings

N/A — not a UI change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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