diff --git a/packages/trpc/src/services/stream.service.test.ts b/packages/trpc/src/services/stream.service.test.ts
new file mode 100644
index 0000000..0233c71
--- /dev/null
+++ b/packages/trpc/src/services/stream.service.test.ts
@@ -0,0 +1,18 @@
+import { describe, expect, it } from "vitest";
+import { resolveContentType } from "./stream.service.js";
+
+describe("resolveContentType", () => {
+    it("keeps existing mime", () => {
+        expect(resolveContentType("video/mp4", "a.mkv")).toBe("video/mp4");
+    });
+    it("maps common video extensions", () => {
+        expect(resolveContentType(null, "/m/a.mp4")).toBe("video/mp4");
+        expect(resolveContentType("", "/m/a.m4v")).toBe("video/mp4");
+        expect(resolveContentType(undefined, "/m/a.mkv")).toBe("video/x-matroska");
+        expect(resolveContentType(null, "/m/a.webm")).toBe("video/webm");
+    });
+    it("falls back to video/<ext> or octet-stream", () => {
+        expect(resolveContentType(null, "/m/a.avi")).toBe("video/avi");
+        expect(resolveContentType(null, "/m/noext")).toBe("application/octet-stream");
+    });
+});
diff --git a/packages/trpc/src/services/stream.service.ts b/packages/trpc/src/services/stream.service.ts
index 6c7d879..3e5e9f2 100644
--- a/packages/trpc/src/services/stream.service.ts
+++ b/packages/trpc/src/services/stream.service.ts
@@ -8,6 +8,23 @@ export type RemoteFileStream = {
     body: ReadableStream<Uint8Array> | null;
 };
 
+const MIME_BY_EXT: Record<string, string> = {
+    mp4: "video/mp4",
+    m4v: "video/mp4",
+    mkv: "video/x-matroska",
+    webm: "video/webm",
+};
+
+/** scan/ensurePath 不写 mime；按扩展名回落，保证播放器拿到可识别的 Content-Type */
+export function resolveContentType(mime: string | null | undefined, path: string): string {
+    if (mime) return mime;
+    const name = path.split("/").pop() ?? path;
+    const dot = name.lastIndexOf(".");
+    const ext = dot >= 0 ? name.slice(dot + 1).toLowerCase() : "";
+    if (!ext) return "application/octet-stream";
+    return MIME_BY_EXT[ext] ?? `video/${ext}`;
+}
+
 type ParsedRange = { start: number; end: number } | "unsatisfiable" | null;
 
 /**
@@ -54,7 +71,7 @@ export async function openRemoteFileStream(
     const abs = mount.rootPath.replace(/\/+$/, "") + item.path;
 
     const headers = new Headers();
-    headers.set("Content-Type", item.mime || "application/octet-stream");
+    headers.set("Content-Type", resolveContentType(item.mime, item.path));
     headers.set("Accept-Ranges", "bytes");
     headers.set("Cache-Control", "private, max-age=0");