fix(gateway): sniff cached audio container type

gateway/platforms/base.py

@@ -844,6 +844,24 @@ def get_audio_cache_dir() -> Path:
     return d
 
 
+def _sniff_audio_ext(data: bytes, fallback_ext: str) -> str:
+    """Prefer a container-matching extension when audio magic bytes are obvious."""
+    fallback = fallback_ext if fallback_ext.startswith(".") else f".{fallback_ext}"
+    if len(data) >= 8 and data[4:8] == b"ftyp":
+        return ".m4a"
+    if data.startswith(b"OggS"):
+        return ".ogg"
+    if data.startswith(b"fLaC"):
+        return ".flac"
+    if len(data) >= 12 and data.startswith(b"RIFF") and data[8:12] == b"WAVE":
+        return ".wav"
+    if data.startswith(b"ID3") or data[:2] in {b"\xff\xfb", b"\xff\xf3", b"\xff\xf2"}:
+        return ".mp3"
+    if data.startswith(b"\x1a\x45\xdf\xa3"):
+        return ".webm"
+    return fallback
+
+
 def cache_audio_from_bytes(data: bytes, ext: str = ".ogg") -> str:
     """
     Save raw audio bytes to the cache and return the absolute file path.
@@ -857,7 +875,8 @@ def cache_audio_from_bytes(data: bytes, ext: str = ".ogg") -> str:
     """
     validate_inbound_media_size(len(data), media_type="audio")
     cache_dir = get_audio_cache_dir()
-    filename = f"audio_{uuid.uuid4().hex[:12]}{ext}"
+    sniffed_ext = _sniff_audio_ext(data, ext)
+    filename = f"audio_{uuid.uuid4().hex[:12]}{sniffed_ext}"
     filepath = cache_dir / filename
     filepath.write_bytes(data)
     return str(filepath)

tests/gateway/test_platform_base.py

@@ -18,6 +18,7 @@ from gateway.platforms.base import (
     validate_inbound_media_size,
     _log_safe_path,
     _prefix_within_utf16_limit,
+    cache_audio_from_bytes,
 )
 
 
@@ -120,6 +121,26 @@ class TestSafeUrlForLog:
         assert safe_url_for_log(url, max_len=0) == ""
 
 
+class TestCacheAudioFromBytes:
+    def test_sniffs_mp4_quicktime_audio_even_when_ext_is_ogg(self, tmp_path):
+        payload = b"\x00\x00\x00\x14ftypqt  " + b"\x00" * 32
+        with patch("gateway.platforms.base.AUDIO_CACHE_DIR", tmp_path):
+            result = cache_audio_from_bytes(payload, ext=".ogg")
+
+        saved = tmp_path / os.path.basename(result)
+        assert saved.suffix == ".m4a"
+        assert saved.read_bytes() == payload
+
+    def test_preserves_fallback_ext_when_audio_header_is_unknown(self, tmp_path):
+        payload = b"not-a-known-audio-header"
+        with patch("gateway.platforms.base.AUDIO_CACHE_DIR", tmp_path):
+            result = cache_audio_from_bytes(payload, ext=".aac")
+
+        saved = tmp_path / os.path.basename(result)
+        assert saved.suffix == ".aac"
+        assert saved.read_bytes() == payload
+
+
 # ---------------------------------------------------------------------------
 # MessageEvent — command parsing
 # ---------------------------------------------------------------------------