- Add ContentType field to FileEntry struct
- Update Set method signature to accept contentType parameter
- Use cached ContentType in static.go cache hit branches
- Update all test files to use new Set signature
This avoids redundant MIME type detection on cache hits,
reducing lock contention in mimeutil.DetectContentType.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>