Remove magic byte detection in favor of simple UTF-8 validation:
- text/plain for valid UTF-8 content
- application/octet-stream for binary data
Security maintained via headers (X-Content-Type-Options: nosniff, CSP).
Magic signatures preserved as comments for future reference.
Disabled test files:
- test_mime_detection.py.disabled (magic-dependent tests)
- test_polyglot.py.disabled (polyglot format tests)
For full MIME detection, consider using the `filetype` library.
