fix(elements): render file elements with null mime instead of crashing

[mihidumh]

Summary

Opening or resuming a thread that contains a persisted file element with a null mime crashes the entire thread view with Cannot read properties of null (reading 'startsWith'). Because the error is thrown during render, the whole conversation becomes unviewable.

This is a regression from #2783 ("add image preview") and is easy to hit with any text-based file (.md, .csv, .txt, source code, …): filetype.guess() detects by magic bytes only, so those files are persisted with mime = NULL.

Fixes #2938

Changes

Frontend (the crash fix)

• Attachment.tsx — mime is now optional and the dereference is guarded: mime.startsWith('image/') → !!mime?.startsWith('image/').
• Elements/File.tsx — dropped the unsafe element.mime! non-null assertion that masked the runtime null.

Backend (defense-in-depth)

• Element.send() — added a filename-based mime fallback (mimetypes.guess_type(path or name)) when filetype.guess() returns None, so text-based files get a sensible mime instead of being persisted as NULL.

Tests

• Added two tests covering the mime fallback for content-based and path-based text files.

Verification

• Backend: pytest tests/test_element.py — 45 passed (incl. 2 new)
• Frontend: tsc --noEmit clean
• Lint/format: ruff, eslint, prettier all clean on changed files

The frontend guard alone repairs already-broken threads (no data backfill required); the backend fallback prevents new text-file elements from being stored with a null mime.

🤖 Generated with Claude Code

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Summary by cubic

Fixes a crash in the thread view when rendering file elements with a null mime. Adds a safe frontend check and a backend mime fallback so existing threads open and new text files get a correct mime.

• Bug Fixes
  • Frontend: Guard mime in Attachment.tsx and remove the non-null assertion in File.tsx.
  • Backend: In Element.send(), fall back to mimetypes.guess_type() when filetype.guess() returns None (covers .md, .csv, .txt, source files).
  • Tests: Added two tests for the filename-based mime fallback (content and path cases).

^{Written for commit 43514ab. Summary will update on new commits.}

Review in cubic

[cubic-dev-ai]

No issues found across 4 files

_{Re-trigger cubic}