add comprehensive MIME detection tests (50 tests)

Cover all 42 magic byte signatures:
- Images: BMP, TIFF, ICO
- Video: MP4, WebM, FLV
- Audio: MP3, FLAC, OGG
- Documents: MS Office OLE
- Executables: PE, ELF, Mach-O, WASM
- Archives: BZIP2, XZ, ZSTD, LZ4, 7z, RAR
- Data: SQLite
- Edge cases: empty, short, boundary tests

Also adds missing Mach-O 32-bit little-endian signature.

README.md

@@ -386,7 +386,7 @@ flaskpaste/
 │   └── api/
 │       ├── __init__.py  # Blueprint setup
 │       └── routes.py    # API endpoints
-├── tests/               # Test suite (337 tests)
+├── tests/               # Test suite (356 tests)
 ├── data/                # SQLite database
 ├── run.py               # Development server
 ├── wsgi.py              # Production WSGI entry