fix(archive): magic-byte archive detection so mis-named archives extract — #713 part 1

Patreon attachment downloads land with sanitized URL-blob filenames
(01_https___www.patreon.com_media-u_v3_<id>) whose Path.suffix is junk, never
.zip — so the extension-only is_archive() filed them as opaque PostAttachments
and never extracted them ("No images attached to this post").

Add archive_extractor.detect_archive_format() — extension first, then magic-byte
sniff (zipfile.is_zipfile + RAR/7z signatures). is_archive(), extract_archive(),
and safe_probe._inspect_archive() (the bomb-guard) all route through it, so a
mis-named/extension-less archive is now detected, bomb-guarded, integrity-tested,
AND extracted regardless of filename. Stops new ones; part 2 re-extracts the
already-imported backlog.

Tests: mis-named zip detected + extracted; non-archive dotted name not
misdetected; _inspect_archive on a mis-named zip; signature updated.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/app/services/archive_extractor.py

@@ -16,9 +16,45 @@ log = logging.getLogger(__name__)
 
 ARCHIVE_EXTS = {".zip", ".cbz", ".rar", ".7z"}
 
+# Magic-byte signatures, so an archive with a mangled / extension-less filename
+# is still recognised. Patreon attachment download URLs sanitize to names like
+# `01_https___www.patreon.com_media-u_v3_131083093`, whose `Path.suffix` is junk
+# (`.com_media-u_v3_131083093`), never `.zip` — an extension-only gate filed
+# those as opaque PostAttachments and NEVER extracted them (operator-flagged
+# 2026-06-06). Detection is by extension first (cheap), then header sniff.
+_RAR_MAGIC = b"Rar!\x1a\x07"
+_7Z_MAGIC = b"7z\xbc\xaf\x27\x1c"
+
+
+def detect_archive_format(path: Path) -> str | None:
+    """Return "zip" | "rar" | "7z" for an archive, else None.
+
+    Trusts a known extension first, then falls back to magic-byte sniffing so a
+    mis-named or extension-less archive is still handled. (zip covers .cbz too.)
+    """
+    ext = Path(path).suffix.lower()
+    if ext in (".zip", ".cbz"):
+        return "zip"
+    if ext == ".rar":
+        return "rar"
+    if ext == ".7z":
+        return "7z"
+    try:
+        if zipfile.is_zipfile(path):
+            return "zip"
+        with open(path, "rb") as fh:
+            head = fh.read(8)
+    except OSError:
+        return None
+    if head.startswith(_RAR_MAGIC):
+        return "rar"
+    if head.startswith(_7Z_MAGIC):
+        return "7z"
+    return None
+
 
 def is_archive(path: Path) -> bool:
-    return Path(path).suffix.lower() in ARCHIVE_EXTS
+    return detect_archive_format(path) is not None
 
 
 @contextmanager
@@ -32,16 +68,16 @@ def extract_archive(path: Path):
     members: list[tuple[str, Path]] = []
     try:
         try:
-            ext = Path(path).suffix.lower()
+            fmt = detect_archive_format(path)
-            if ext in (".zip", ".cbz"):
+            if fmt == "zip":
                 with zipfile.ZipFile(path) as zf:
                     zf.extractall(base)
-            elif ext == ".rar":
+            elif fmt == "rar":
                 import rarfile
 
                 with rarfile.RarFile(path) as rf:
                     rf.extractall(base)
-            elif ext == ".7z":
+            elif fmt == "7z":
                 import py7zr
 
                 with py7zr.SevenZipFile(path, "r") as zf:

backend/app/utils/safe_probe.py

@@ -149,9 +149,8 @@ def _run_probe(path_str: str) -> tuple[str, str | None]:
     call the same code path.
     """
     path = Path(path_str)
-    ext = path.suffix.lower()
     try:
-        total, test_bad = _inspect_archive(path, ext)
+        total, test_bad = _inspect_archive(path)
     except Exception as exc:  # noqa: BLE001 — clean rejection
         return ("error", f"{type(exc).__name__}: {exc}")
     if total is not None and total > MAX_ARCHIVE_UNCOMPRESSED_BYTES:
@@ -162,24 +161,29 @@ def _run_probe(path_str: str) -> tuple[str, str | None]:
     return ("ok", None)
 
 
-def _inspect_archive(path: Path, ext: str):
+def _inspect_archive(path: Path):
     """Return (total_uncompressed_bytes | None, first_bad_member | None)
-    for the archive. Format-specific; raises on a structurally-broken
+    for the archive. Format detected by extension OR magic bytes (so a
-    container (caught by the child as a clean rejection)."""
+    mis-named archive is still bomb-guarded + integrity-tested, matching the
-    if ext in (".zip", ".cbz"):
+    extractor's gate); raises on a structurally-broken container (caught by the
+    child as a clean rejection)."""
+    from ..services.archive_extractor import detect_archive_format
+
+    fmt = detect_archive_format(path)
+    if fmt == "zip":
         import zipfile
 
         with zipfile.ZipFile(path) as zf:
             total = sum(zi.file_size for zi in zf.infolist())
             return total, zf.testzip()
-    if ext == ".rar":
+    if fmt == "rar":
         import rarfile
 
         with rarfile.RarFile(path) as rf:
             total = sum(getattr(ri, "file_size", 0) for ri in rf.infolist())
             rf.testrar()
             return total, None
-    if ext == ".7z":
+    if fmt == "7z":
         import py7zr
 
         with py7zr.SevenZipFile(path, "r") as zf:
@@ -187,5 +191,5 @@ def _inspect_archive(path: Path, ext: str):
             total = getattr(info, "uncompressed", None)
             ok = zf.test()  # True / None when all members pass
             return total, (None if ok in (True, None) else "7z test reported corruption")
-    # Unknown extension — nothing to test; treat as clean.
+    # Not a recognised archive — nothing to test; treat as clean.
     return None, None

tests/test_archive_extractor.py

@@ -39,6 +39,24 @@ def test_cbz_alias(tmp_path):
         assert [n for n, _ in members] == ["p1.jpg"]
 
 
+def test_misnamed_zip_detected_and_extracted(tmp_path):
+    """A real zip whose filename has no usable extension (Patreon attachment
+    URL-blob name) is detected by magic bytes and its members extracted —
+    previously it was filed as an opaque attachment and never opened."""
+    z = tmp_path / "01_https___www.patreon.com_media-u_v3_131083093"
+    _zip(z, {"a.jpg": b"img", "b.png": b"img2"})
+    assert is_archive(z)
+    with extract_archive(z) as members:
+        assert sorted(n for n, _ in members) == ["a.jpg", "b.png"]
+
+
+def test_non_archive_with_dotted_name_is_not_archive(tmp_path):
+    """A non-archive file with a dotted/extension-less name isn't misdetected."""
+    f = tmp_path / "01_https___www.patreon.com_media-u_v3_999"
+    f.write_bytes(b"\x89PNG\r\n\x1a\n not really but not a zip")
+    assert not is_archive(f)
+
+
 def test_corrupt_archive_yields_nothing(tmp_path):
     bad = tmp_path / "broken.zip"
     bad.write_bytes(b"not a zip at all")

tests/test_safe_probe.py

@@ -40,11 +40,21 @@ def test_probe_archive_corrupt_zip_clean_rejection(tmp_path):
 def test_inspect_archive_reports_size_and_clean_integrity(tmp_path):
     z = tmp_path / "sized.zip"
     _zip(z, {"a.txt": b"x" * 100, "b.txt": b"y" * 50})
-    total, bad = safe_probe._inspect_archive(z, ".zip")
+    total, bad = safe_probe._inspect_archive(z)
     assert total == 150
     assert bad is None
 
 
+def test_inspect_archive_detects_misnamed_zip(tmp_path):
+    """A zip with a mangled / extension-less name (Patreon URL-blob attachment)
+    is still bomb-guarded + integrity-tested via magic-byte detection."""
+    z = tmp_path / "01_https___www.patreon.com_media-u_v3_131083093"
+    _zip(z, {"a.txt": b"x" * 100})
+    total, bad = safe_probe._inspect_archive(z)
+    assert total == 100
+    assert bad is None
+
+
 def test_run_probe_bomb_guard(tmp_path, monkeypatch):
     """In-process call to _run_probe so the monkeypatched cap takes effect.
     The public probe_archive runs in a subprocess which re-imports the