simplify MIME detection to text/binary only

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.

tests/test_abuse_prevention.py

@@ -296,24 +296,26 @@ class TestBinaryRequirement:
         assert data["detected"] == "text/plain"
         assert "hint" in data
 
-    def test_png_rejected(self, binary_client):
-        """PNG magic bytes should be rejected."""
+    def test_png_accepted_as_binary(self, binary_client):
+        """PNG content accepted as unrecognized binary (magic detection disabled)."""
         # PNG signature: 89 50 4E 47 0D 0A 1A 0A
         png_content = b"\x89PNG\r\n\x1a\n" + b"\x00" * 100
         response = binary_client.post("/", data=png_content)
-        assert response.status_code == 400
+        # With magic detection disabled, PNG bytes are just binary
+        assert response.status_code == 201
 
         data = response.get_json()
-        assert data["detected"] == "image/png"
+        assert data["mime_type"] == "application/octet-stream"
 
-    def test_jpeg_rejected(self, binary_client):
-        """JPEG magic bytes should be rejected."""
+    def test_jpeg_accepted_as_binary(self, binary_client):
+        """JPEG content accepted as unrecognized binary (magic detection disabled)."""
         jpeg_content = b"\xff\xd8\xff" + b"\x00" * 100
         response = binary_client.post("/", data=jpeg_content)
-        assert response.status_code == 400
+        # With magic detection disabled, JPEG bytes are just binary
+        assert response.status_code == 201
 
         data = response.get_json()
-        assert data["detected"] == "image/jpeg"
+        assert data["mime_type"] == "application/octet-stream"
 
     def test_random_binary_accepted(self, binary_client):
         """Random binary data (encrypted) should be accepted."""