diff --git a/src/polygateway/transports/monkey_ocr.py b/src/polygateway/transports/monkey_ocr.py
index 9e0edc9..254cc55 100644
--- a/src/polygateway/transports/monkey_ocr.py
+++ b/src/polygateway/transports/monkey_ocr.py
@@ -14,7 +14,7 @@ import math
 import zipfile
 from contextlib import contextmanager
 from io import BytesIO
-from typing import TYPE_CHECKING, Any
+from typing import TYPE_CHECKING, Any, Literal
 
 import httpx
 
@@ -42,6 +42,50 @@ if TYPE_CHECKING:
 
 _HEALTH_TIMEOUT_S = 5.0  # 探测常量(同 jitter 系数先例,不进配置)
 
+_MediaKind = Literal["pdf", "image", "unknown"]
+
+# 已知图片魔数。起始命中即判图片,且**优先于** PDF 判定: 含 XMP 元数据的图片
+# 可能在前 1KB 内带 `%PDF-` 字样,不能因此被当成 PDF。
+_IMAGE_MAGICS: tuple[bytes, ...] = (
+    b"\xff\xd8\xff",  # JPEG
+    b"\x89PNG\r\n\x1a\n",  # PNG
+    b"II*\x00",  # TIFF 小端
+    b"MM\x00*",  # TIFF 大端
+    b"BM",  # BMP
+    b"GIF87a",
+    b"GIF89a",
+)
+_PDF_HEADER = b"%PDF-"
+# Adobe 阅读器容忍头部前有垃圾字节(惯例而非 ISO 32000 正文);服务端实测可解析
+# 前缀 200 字节垃圾的 PDF(#34 设计 E3)
+_PDF_HEADER_WINDOW = 1024
+
+
+def _sniff_media(data: bytes) -> _MediaKind:
+    """按内容判定上传类型: 图片魔数 → 前 1KB 内 `%PDF-` → 未知。
+
+    服务端按**文件名扩展名**分派(`.pdf` 走 PDF 数据集,其余走图片数据集),而标签
+    若与内容不符会得到 HTTP 500(`is no PDF`)或——在 `/ocr/text` 上——`success:true`
+    的幻觉文本(#34 设计 E2/E4)。所以标签只能由库按内容定,不开放给调用方声明。
+    """
+    if data.startswith(_IMAGE_MAGICS) or (data[:4] == b"RIFF" and data[8:12] == b"WEBP"):
+        return "image"
+    if _PDF_HEADER in data[:_PDF_HEADER_WINDOW]:
+        return "pdf"
+    return "unknown"
+
+
+def _upload_part(data: bytes) -> tuple[str, bytes, str]:
+    """multipart 的 (filename, content, content_type)。
+
+    只有 PDF 改标 `doc.pdf`;**其余(含未知)一律沿用历史的 `image.jpg`**——服务对
+    PNG/TIFF/BMP 字节标 `image.jpg` 照样按内容解码(2026-10-21 实测),任何"更精确"
+    的改标都只会把现在能成功的路径变成失败(webp/gif 没有对应的服务端扩展名)。
+    """
+    if _sniff_media(data) == "pdf":
+        return ("doc.pdf", data, "application/pdf")
+    return ("image.jpg", data, "image/jpeg")
+
 
 def _default_client_factory(source: SourceConfig) -> httpx.AsyncClient:
     return httpx.AsyncClient(
@@ -205,9 +249,10 @@ class MonkeyOcrTransport:
         return client
 
     @staticmethod
-    def _files(image: bytes) -> dict[str, tuple[str, bytes, str]]:
-        # 固定文件名/类型(CHS invokers.py:496 原样;服务按内容处理,35 样本实测)
-        return {"file": ("image.jpg", image, "image/jpeg")}
+    def _files(data: bytes) -> dict[str, tuple[str, bytes, str]]:
+        # 标签按内容判定(#34): 非 PDF 沿用 CHS invokers.py:496 的 image.jpg,
+        # PDF 改标 doc.pdf——服务按扩展名分派,固定名对 PDF 是错的
+        return {"file": _upload_part(data)}
 
     def _response_json(self, resp: httpx.Response, source: SourceConfig, operation: str) -> dict:
         """响应体 JSON 化;坏 JSON/非 dict → Transient(设计 §4: 服务侧瞬时异常)。"""
diff --git a/tests/unit/test_monkey_ocr.py b/tests/unit/test_monkey_ocr.py
index f0c100e..21ae776 100644
--- a/tests/unit/test_monkey_ocr.py
+++ b/tests/unit/test_monkey_ocr.py
@@ -23,6 +23,8 @@ from polygateway.transports.monkey_ocr import (
     MonkeyOcrTransport,
     _classify_status,
     _parse_middle_json,
+    _sniff_media,
+    _upload_part,
 )
 from polygateway.types import SourceConfig
 
@@ -167,6 +169,86 @@ class TestRecognizeText:
         assert b'filename="image.jpg"' in seen["body"]  # CHS invokers.py:496 固定名
 
 
+_PDF_BODY = b"%PDF-1.4\n%synthetic body"
+_JUNK = b"x" * 200
+
+
+class TestSniffMedia:
+    """上传类型按内容判定(#34 设计 §3 Q2): 图片魔数优先 → 前 1KB 内 `%PDF-` → 未知。"""
+
+    @pytest.mark.parametrize(
+        ("data", "expected"),
+        [
+            pytest.param(_PDF_BODY, "pdf", id="pdf"),
+            pytest.param(_JUNK + _PDF_BODY, "pdf", id="pdf-with-junk-prefix"),
+            pytest.param(b"\xff\xd8\xff\xe0" + b"0" * 20, "image", id="jpeg"),
+            pytest.param(b"\x89PNG\r\n\x1a\n" + b"0" * 20, "image", id="png"),
+            pytest.param(b"II*\x00" + b"0" * 20, "image", id="tiff-le"),
+            pytest.param(b"MM\x00*" + b"0" * 20, "image", id="tiff-be"),
+            pytest.param(b"BM" + b"0" * 20, "image", id="bmp"),
+            pytest.param(b"GIF89a" + b"0" * 20, "image", id="gif89a"),
+            pytest.param(b"GIF87a" + b"0" * 20, "image", id="gif87a"),
+            pytest.param(b"RIFF\x00\x00\x00\x00WEBPVP8 ", "image", id="webp"),
+            # 图片魔数优先: 含 XMP 元数据的图片可能在 1KB 内带 `%PDF-` 字样
+            pytest.param(b"\xff\xd8\xff\xe0" + b"<xmp>%PDF-</xmp>", "image", id="jpeg-mentions-pdf"),
+            pytest.param(b"<html><body>paywall</body></html>", "unknown", id="html"),
+            pytest.param(b"GARBAGE-NOT-AN-IMAGE", "unknown", id="garbage"),
+            pytest.param(b"", "unknown", id="empty"),
+            # 1KB 窗口之外的 `%PDF-` 不算
+            pytest.param(b"x" * 1100 + b"%PDF-", "unknown", id="header-beyond-window"),
+        ],
+    )
+    def test_sniff(self, data, expected):
+        assert _sniff_media(data) == expected
+
+    def test_upload_part_labels_pdf_only(self):
+        assert _upload_part(_PDF_BODY) == ("doc.pdf", _PDF_BODY, "application/pdf")
+        assert _upload_part(_JUNK + _PDF_BODY)[0] == "doc.pdf"
+
+    @pytest.mark.parametrize("data", [b"\xff\xd8\xff\xe0abc", b"\x89PNG\r\n\x1a\nabc", b"junk", b""])
+    def test_everything_else_keeps_the_legacy_label(self, data):
+        """非 PDF 路径字节级不变: 服务对图片按内容解码(2026-10-21 实测 PNG/TIFF/BMP 标 image.jpg 成功)。"""
+        assert _upload_part(data) == ("image.jpg", data, "image/jpeg")
+
+
+class TestUploadLabelPerMethod:
+    """三个调用点共用同一个标签点: 漏掉任何一个,E4 的静默错误结果就在那里复活。"""
+
+    @staticmethod
+    def _capturing_transport(seen):
+        def handler(request: httpx.Request) -> httpx.Response:
+            if request.url.path == "/ocr/text":
+                seen["body"] = request.read()
+                return httpx.Response(200, json=_text_body())
+            if request.url.path == "/parse":
+                seen["body"] = request.read()
+                return httpx.Response(200, json=_parse_body())
+            return httpx.Response(200, content=_zip_bytes())
+
+        return _transport_for(handler)
+
+    async def test_recognize_text_labels_pdf_as_pdf(self):
+        seen = {}
+        t = self._capturing_transport(seen)
+        await t.recognize_text(image=_PDF_BODY, source=_source(), call_id="c1")
+        assert b'filename="doc.pdf"' in seen["body"]
+        assert b"Content-Type: application/pdf" in seen["body"]
+
+    async def test_parse_layout_labels_pdf_as_pdf(self):
+        seen = {}
+        t = self._capturing_transport(seen)
+        await t.parse_layout(image=_PDF_BODY, source=_source(), call_id="c1")
+        assert b'filename="doc.pdf"' in seen["body"]
+        assert b"Content-Type: application/pdf" in seen["body"]
+
+    async def test_parse_layout_keeps_image_label_for_images(self):
+        seen = {}
+        t = self._capturing_transport(seen)
+        await t.parse_layout(image=b"\xff\xd8\xff\xe0abc", source=_source(), call_id="c1")
+        assert b'filename="image.jpg"' in seen["body"]
+        assert b"Content-Type: image/jpeg" in seen["body"]
+
+
 class TestParseLayout:
     async def test_full_chain_relative_url(self):
         t = _transport_for(