diff --git a/tests/test_attachments_pdf_audio.py b/tests/test_attachments_pdf_audio.py
index cebdcdf2..9174151d 100644
--- a/tests/test_attachments_pdf_audio.py
+++ b/tests/test_attachments_pdf_audio.py
@@ -30,6 +30,9 @@ FLAC = b"fLaC\x00\x00\x00\x22" + b"\x00" * 8
 M4A = b"\x00\x00\x00\x20ftypM4A \x00\x00\x00\x00"
 # Major brand mp42 but M4A in the compatible-brands list (common for real .m4a).
 M4A_COMPAT = b"\x00\x00\x00\x20ftypmp42\x00\x00\x00\x00M4A mp42isom"
+# M4A as a LATE compatible brand (offset 40), past the old fixed 16:40 scan window
+# — must still be accepted now that the whole ftyp box is scanned.
+M4A_LATE_BRAND = b"\x00\x00\x00\x2cftypmp42\x00\x00\x00\x00isomiso2mp41avc1dashmp4aM4A "
 # ISO-BMFF *video* / MOV share the ftyp box — must NOT sniff as audio.
 MP4_VIDEO = b"\x00\x00\x00\x20ftypisom\x00\x00\x02\x00isomiso2avc1mp41"
 MOV_VIDEO = b"\x00\x00\x00\x14ftypqt  \x00\x00\x00\x00qt  \x00\x00\x00\x00"
@@ -60,6 +63,7 @@ class TestSniffAudio:
         assert sniff_audio_mime(FLAC) == "audio/flac"
         assert sniff_audio_mime(M4A) == "audio/mp4"
         assert sniff_audio_mime(M4A_COMPAT) == "audio/mp4"
+        assert sniff_audio_mime(M4A_LATE_BRAND) == "audio/mp4"
         assert sniff_audio_mime(AAC_ADTS) == "audio/aac"
         assert sniff_audio_mime(WEBM) == "audio/webm"
 
diff --git a/turnstone/core/attachments.py b/turnstone/core/attachments.py
index 106e3bc1..00083f5a 100644
--- a/turnstone/core/attachments.py
+++ b/turnstone/core/attachments.py
@@ -181,14 +181,20 @@ def sniff_audio_mime(data: bytes) -> str | None:
     if data[:4] == b"fLaC":
         return "audio/flac"
     # ISO-BMFF: the ``ftyp`` box is shared by MP4/MOV *video* and M4A/M4B
-    # *audio*.  Accept only audio brands (major, or an audio brand in the
-    # compatible-brands list) so a video file can't masquerade as audio.
-    if data[4:8] == b"ftyp" and (
-        data[8:12] in (b"M4A ", b"M4B ", b"F4A ", b"F4B ")
-        or b"M4A " in data[16:40]
-        or b"M4B " in data[16:40]
-    ):
-        return "audio/mp4"
+    # *audio*.  Accept only when an audio brand is the major brand or appears
+    # anywhere in the compatible-brands list — scan the whole ftyp box (its
+    # length is the big-endian uint32 at data[0:4]) so a real .m4a with the
+    # audio brand listed late still passes; a pure-video file carries none.  A
+    # generic-MP4 (isom/mp42) audio stream with no audio brand is
+    # indistinguishable from video by magic bytes alone, so it falls through
+    # (rejected) rather than risk passing a video off as audio.
+    if data[4:8] == b"ftyp":
+        box_end = int.from_bytes(data[0:4], "big")
+        if not 16 <= box_end <= len(data):
+            box_end = len(data)
+        brands = data[8:box_end]  # major brand (8:12) + compatible brands (16:end)
+        if any(b in brands for b in (b"M4A ", b"M4B ", b"F4A ", b"F4B ")):
+            return "audio/mp4"
     if data[:4] == b"\x1aE\xdf\xa3":
         return "audio/webm"
     return None
diff --git a/turnstone/core/session.py b/turnstone/core/session.py
index a603dafe..046123d0 100644
--- a/turnstone/core/session.py
+++ b/turnstone/core/session.py
@@ -2969,7 +2969,9 @@ class ChatSession:
         # re-rasterized / a blob re-base64'd once per round-trip.  Key on
         # (id, caps-signature): the same stored blob materializes differently per
         # capability set, and a fallback to a different-caps model can resolve
-        # within one send.  ``cache`` is None outside a send → original behavior.
+        # within one send.  The cache is refreshed per send (set in send()); the
+        # wire resolver runs only during a send, so a stale value between sends is
+        # never read.  A None cache disables memoization (the original behavior).
         cache = self._wire_part_cache
         caps_sig = (caps.supports_pdf, caps.supports_vision, caps.supports_audio_input)
         out: dict[str, Any] = {}
diff --git a/turnstone/shared_static/composer_attachments.js b/turnstone/shared_static/composer_attachments.js
index 1d28ffc1..5edb7053 100644
--- a/turnstone/shared_static/composer_attachments.js
+++ b/turnstone/shared_static/composer_attachments.js
@@ -125,20 +125,31 @@ export function buildAttachmentPreview(opts) {
     })
       .then(function (r) {
         if (!r || !r.ok) return "";
-        // We only render ~240 chars: read just the first body chunk and cancel
-        // the rest of the transfer instead of downloading the whole blob (text
-        // attachments are capped at 512 KiB, ~2000x the rendered size).  The
-        // first network chunk is always many KB, so 240 chars are available.
+        // We only render ~240 chars: read body chunks until we have enough, then
+        // cancel the rest of the transfer instead of downloading the whole blob
+        // (text attachments are capped at 512 KiB, ~2000x the rendered size).
+        // Accumulate rather than assume one chunk ≥ 240 chars (flush boundaries
+        // can split a large body into small early chunks).
         if (!r.body || typeof r.body.getReader !== "function") return r.text();
         var reader = r.body.getReader();
-        return reader.read().then(function (res) {
-          try {
-            reader.cancel();
-          } catch (e) {
-            /* already closed */
-          }
-          return res && res.value ? new TextDecoder().decode(res.value) : "";
-        });
+        var dec = new TextDecoder();
+        var acc = "";
+        function pump() {
+          return reader.read().then(function (res) {
+            if (res && res.value)
+              acc += dec.decode(res.value, { stream: true });
+            if (res.done || acc.length >= 240) {
+              try {
+                reader.cancel();
+              } catch (e) {
+                /* already closed */
+              }
+              return acc;
+            }
+            return pump();
+          });
+        }
+        return pump();
       })
       .then(function (t) {
         if (!t) {