Summary

• Anchor the MIME type sanitization regex with $ to reject trailing content after a valid type/subtype pair
• Add NFKC Unicode normalization before validation to prevent fullwidth character bypasses (e.g., ａｕｄｉｏ/ｍｐｅｇ)
• Apply the same normalization to normalizeMimeType() in src/media/input-files.ts

Fixes #9791, #9795

Test plan

• New tests for sanitizeMimeType() covering: standard types, fullwidth Unicode, trailing content, invalid values
• New tests for normalizeMimeType() covering: charset stripping, fullwidth Unicode normalization
• All 11 new tests pass
• pnpm check passes (0 warnings, 0 errors)

Greptile Overview

Greptile Summary

• Updates MIME sanitization in src/media-understanding/apply.ts to NFKC-normalize before validation and anchors the regex to reject trailing content.
• Exports sanitizeMimeType() and adds targeted Vitest coverage for standard types, whitespace/lowercasing, fullwidth Unicode normalization, and trailing-content rejection.
• Updates normalizeMimeType() in src/media/input-files.ts to also apply NFKC normalization, with tests covering parameter/charset stripping and fullwidth normalization.
• Change fits into the media ingestion pipeline by hardening MIME handling before allowing/denying extraction and before embedding MIME values into generated <file ...> blocks.

Confidence Score: 5/5

• This PR is safe to merge with minimal risk.
• Changes are tightly scoped to MIME normalization/validation, include explicit regression tests for the intended security fixes (trailing-content rejection and fullwidth bypass prevention), and do not alter unrelated control flow in the media pipeline.
• No files require special attention