add Hypothesis property-based MIME detection tests

- test_magic_prefix_detection: verify all signatures with random suffix
- test_random_binary_never_crashes: random data never crashes
- test_partial_magic_no_false_match: truncated magic handled safely
- test_magic_not_at_start_ignored: only detect magic at offset 0

tests/test_fuzz.py

@@ -168,6 +168,108 @@ class TestMimeTypeFuzzing:
             pass
 
 
+class TestMimeDetectionFuzzing:
+    """Property-based tests for MIME magic byte detection."""
+
+    # Known magic signatures mapped to expected MIME types
+    MAGIC_SIGNATURES: ClassVar[list[tuple[bytes, str]]] = [
+        (b"\x89PNG\r\n\x1a\n", "image/png"),
+        (b"\xff\xd8\xff", "image/jpeg"),
+        (b"GIF87a", "image/gif"),
+        (b"GIF89a", "image/gif"),
+        (b"%PDF", "application/pdf"),
+        (b"PK\x03\x04", "application/zip"),
+        (b"\x1f\x8b", "application/gzip"),
+        (b"fLaC", "audio/flac"),
+        (b"OggS", "audio/ogg"),
+        (b"ID3", "audio/mpeg"),
+        (b"\x7fELF", "application/x-executable"),
+        (b"MZ", "application/x-msdownload"),
+        (b"BZh", "application/x-bzip2"),
+        (b"7z\xbc\xaf\x27\x1c", "application/x-7z-compressed"),
+        (b"SQLite format 3\x00", "application/x-sqlite3"),
+        # HEIC/HEIF/AVIF (ftyp box format)
+        (b"\x00\x00\x00\x18\x66\x74\x79\x70\x68\x65\x69\x63", "image/heic"),
+        (b"\x00\x00\x00\x18\x66\x74\x79\x70\x6d\x69\x66\x31", "image/heif"),
+        (b"\x00\x00\x00\x1c\x66\x74\x79\x70\x61\x76\x69\x66", "image/avif"),
+    ]
+
+    @settings(max_examples=100, suppress_health_check=FIXTURE_HEALTH_CHECKS)
+    @given(suffix=st.binary(min_size=0, max_size=1000))
+    def test_magic_prefix_detection(self, client, suffix):
+        """Magic bytes followed by arbitrary data should detect correctly."""
+        for magic, expected_mime in self.MAGIC_SIGNATURES:
+            content = magic + suffix
+            response = client.post(
+                "/",
+                data=content,
+                content_type="application/octet-stream",
+            )
+            if response.status_code == 201:
+                data = json.loads(response.data)
+                assert data["mime_type"] == expected_mime, (
+                    f"Expected {expected_mime} for magic {magic!r}, got {data['mime_type']}"
+                )
+
+    @settings(max_examples=200, suppress_health_check=FIXTURE_HEALTH_CHECKS)
+    @given(content=st.binary(min_size=1, max_size=5000))
+    def test_random_binary_never_crashes(self, client, content):
+        """Random binary content should never crash MIME detection."""
+        response = client.post(
+            "/",
+            data=content,
+            content_type="application/octet-stream",
+        )
+        assert response.status_code in (201, 400, 413, 429, 503)
+        if response.status_code == 201:
+            data = json.loads(response.data)
+            # MIME type should always be a valid format
+            assert "/" in data["mime_type"]
+            assert len(data["mime_type"]) < 100
+
+    @settings(max_examples=100, suppress_health_check=FIXTURE_HEALTH_CHECKS)
+    @given(
+        magic=st.sampled_from([m for m, _ in MAGIC_SIGNATURES]),
+        truncate=st.integers(min_value=1, max_value=10),
+    )
+    def test_partial_magic_no_false_match(self, client, magic, truncate):
+        """Truncated magic bytes should not produce false positive matches."""
+        if truncate >= len(magic):
+            return  # Skip if we'd use full magic
+        partial = magic[:truncate]
+        # Add random suffix that's clearly not the rest of the magic
+        content = partial + b"\xff\xfe\xfd\xfc" * 10
+
+        response = client.post(
+            "/",
+            data=content,
+            content_type="application/octet-stream",
+        )
+        # Partial magic should not crash - may match different signature or fallback
+        assert response.status_code in (201, 400, 413, 429, 503)
+
+    @settings(max_examples=50, suppress_health_check=FIXTURE_HEALTH_CHECKS)
+    @given(
+        content=st.binary(min_size=100, max_size=1000),
+        inject_pos=st.integers(min_value=20, max_value=80),
+    )
+    def test_magic_not_at_start_ignored(self, client, content, inject_pos):
+        """Magic bytes not at offset 0 should not trigger detection."""
+        # Inject PNG magic in middle of random data
+        png_magic = b"\x89PNG\r\n\x1a\n"
+        if inject_pos < len(content):
+            modified = content[:inject_pos] + png_magic + content[inject_pos:]
+            response = client.post(
+                "/",
+                data=modified,
+                content_type="application/octet-stream",
+            )
+            if response.status_code == 201:
+                data = json.loads(response.data)
+                # Should NOT detect as PNG (magic not at start)
+                assert data["mime_type"] != "image/png"
+
+
 class TestJsonFuzzing:
     """Fuzz JSON endpoint handling."""