IAV-846/feat(file-manager): add DOCX support and refactor content type resolution

Summary

Extends FileMetadataHelper to support additional file types and refactors the content type resolution logic for better maintainability and correctness.

Changes

• Replaced if/else chain in determineFileType() with a static immutable Map
• Added support for video/mp4 → FileType.VIDEO
• Added support for application/vnd.openxmlformats-officedocument.wordprocessingml.document → FileType.DOCX
• Added isBlank() guard alongside null check on content type input
• Removed default branch from enum switch to enforce exhaustive matching at compile time
• Clarified DOCUMENT case as a generic fallback for unknown document formats

Notes

• FileType.DOCUMENT and FileType.DOCX are intentionally kept separate: DOCX maps to its specific MIME type, while DOCUMENT serves as a generic fallback (application/octet-stream) for unrecognized document formats
• Any future addition to FileType enum will cause a compile-time error if not handled in the switch, preventing silent regressions

Closes IAV-846