diff --git a/app/chat-sessions/page.tsx b/app/chat-sessions/page.tsx
index 61cb8f2..88bda9b 100644
--- a/app/chat-sessions/page.tsx
+++ b/app/chat-sessions/page.tsx
@@ -58,6 +58,7 @@ import { MarkdownRenderer } from "@/components/MarkdownRenderer";
 import { FilePreviewDialog } from "@/components/FilePreviewDialog";
 import { getFileUrl } from "@/lib/ossUpload";
 import {
+  attachmentPreviewUrl,
   sessionAttachmentMime,
   type SessionAttachment,
 } from "@/lib/sessionAttachments";
@@ -873,7 +874,7 @@ export default function ChatSessionsPage() {
                                                         filename: attachment.filename,
                                                         originalname: attachment.filename,
                                                         mimetype: sessionAttachmentMime(attachment),
-                                                        sourceUrl: getFileUrl(attachment.objectKey),
+                                                        sourceUrl: attachmentPreviewUrl(getFileUrl(attachment.objectKey)),
                                                       })
                                                     }
                                                   >
diff --git a/lib/sessionAttachments.ts b/lib/sessionAttachments.ts
index cccb915..36f5152 100644
--- a/lib/sessionAttachments.ts
+++ b/lib/sessionAttachments.ts
@@ -56,12 +56,27 @@ export function normalizeSessionAttachments(raw: unknown): SessionAttachment[] {
   return out;
 }
 
-/** 预览对话框要 MIME；上传时浏览器可能给空串，用扩展名补。 */
+/** content_type 有时是给气泡看的标签（"PDF"），不是 MIME。 */
+function isMimeType(value: string): boolean {
+  return /^[\w.+-]+\/[\w.+-]+$/.test(value);
+}
+
+/**
+ * 对象存储的签名响应带 Content-Disposition: attachment。
+ * iframe 直接跟着 302 走会下载，预览要改走同域 inline 流。
+ */
+export function attachmentPreviewUrl(url: string): string {
+  if (!url.startsWith("/api/oss/")) return url;
+  return `${url}${url.includes("?") ? "&" : "?"}inline=1`;
+}
+
+/** 预览对话框要 MIME；标签、空串、octet-stream 都按扩展名补。 */
 export function sessionAttachmentMime(attachment: SessionAttachment): string {
-  if (attachment.contentType && attachment.contentType !== "application/octet-stream") {
-    return attachment.contentType;
+  const stored = (attachment.contentType || "").split(";")[0].trim();
+  if (stored && stored !== "application/octet-stream" && isMimeType(stored)) {
+    return stored;
   }
   const name = attachment.filename.toLowerCase();
   const ext = Object.keys(MIME_BY_EXT).find((suffix) => name.endsWith(suffix));
-  return (ext && MIME_BY_EXT[ext]) || attachment.contentType || "application/octet-stream";
+  return (ext && MIME_BY_EXT[ext]) || stored || "application/octet-stream";
 }
diff --git a/pages/api/oss/[...key].ts b/pages/api/oss/[...key].ts
index 69a9892..ea764b0 100644
--- a/pages/api/oss/[...key].ts
+++ b/pages/api/oss/[...key].ts
@@ -1,3 +1,4 @@
+import { Readable } from "node:stream";
 import type { NextApiRequest, NextApiResponse } from "next";
 import { getUserIdFromRequest } from "@/lib/auth";
 import { ossClient } from "@/lib/ossClient";
@@ -26,8 +27,25 @@ export default async function handler(req: NextApiRequest, res: NextApiResponse)
   }
 
   try {
-    const { url } = await ossClient.sign({ objectKey });
+    // 签名 URL 上的 Content-Disposition 是 attachment。预览 iframe 跟着 302
+    // 会触发下载，所以 inline=1 时由我们把字节流回去并改成 inline。
+    if (req.query.inline === "1") {
+      const { url } = await ossClient.sign({ objectKey, expiresIn: 600 });
+      const upstream = await fetch(url);
+      if (!upstream.ok || !upstream.body) {
+        return res.status(502).json({ error: "Failed to fetch file" });
+      }
+      const contentType = (upstream.headers.get("content-type") || "application/octet-stream")
+        .split(";")[0]
+        .trim();
+      res.setHeader("Content-Type", contentType || "application/octet-stream");
+      res.setHeader("Content-Disposition", "inline");
+      res.setHeader("Cache-Control", "private, max-age=600");
+      Readable.fromWeb(upstream.body).pipe(res);
+      return;
+    }
 
+    const { url } = await ossClient.sign({ objectKey });
     return res.redirect(302, url);
   } catch (error: any) {
     console.error("[OSS Download] Error:", {
diff --git a/test/sessionAttachments.test.ts b/test/sessionAttachments.test.ts
index 0d20aad..22ebcd9 100644
--- a/test/sessionAttachments.test.ts
+++ b/test/sessionAttachments.test.ts
@@ -3,6 +3,7 @@ import fs from "node:fs";
 import path from "node:path";
 import { test } from "node:test";
 import {
+  attachmentPreviewUrl,
   normalizeSessionAttachments,
   sessionAttachmentMime,
 } from "../lib/sessionAttachments.ts";
@@ -47,6 +48,25 @@ test("空值和非法输入都当成没有附件", () => {
   assert.deepEqual(normalizeSessionAttachments({}), []);
 });
 
+test("会话里记下的是展示标签时，预览仍按扩展名识别 PDF", () => {
+  assert.equal(
+    sessionAttachmentMime({
+      filename: "双中心线试验圆角.pdf",
+      objectKey: "attachments/202609/双中心线试验圆角_1f7749.pdf",
+      contentType: "PDF",
+    }),
+    "application/pdf"
+  );
+});
+
+test("会话附件预览走 inline，避免对象存储的 attachment 头触发下载", () => {
+  assert.equal(
+    attachmentPreviewUrl("/api/oss/attachments/202609/a.pdf"),
+    "/api/oss/attachments/202609/a.pdf?inline=1"
+  );
+  assert.equal(attachmentPreviewUrl("https://example.com/a.pdf"), "https://example.com/a.pdf");
+});
+
 test("预览用 MIME：有有效类型就用，否则按扩展名补", () => {
   assert.equal(
     sessionAttachmentMime({