fix(serve): don't set default content-type for BufferSource response bodies

Per the Fetch spec, BufferSource bodies (ArrayBuffer, TypedArray, DataView)
should not have a default Content-Type header. Previously, Bun.serve()
was injecting `content-type: application/octet-stream` for these bodies
because `getContentType` checked `blob.contentType().len > 0` which
always returned true (InternalBlob defaults to MimeType.other). Changed
to use `blob.hasContentTypeFromUser()` so only user-specified content
types are applied.

Closes #21193

Co-Authored-By: Claude <noreply@anthropic.com>

src/bun.js/api/server/RequestContext.zig

@@ -2639,16 +2639,18 @@ fn getContentType(headers: ?*WebCore.FetchHeaders, blob: *const WebCore.Blob.Any
             }
         }
 
-        break :brk if (blob.contentType().len > 0)
+        break :brk if (blob.hasContentTypeFromUser())
             MimeType.byName(blob.contentType())
         else if (MimeType.sniff(blob.slice())) |content|
             content
         else if (blob.wasString())
             MimeType.text
-                // TODO: should we get the mime type off of the Blob.Store if it exists?
-                // A little wary of doing this right now due to causing some breaking change
-        else
-            MimeType.other;
+        else {
+            // Per the Fetch spec, BufferSource bodies (ArrayBuffer, TypedArray, DataView)
+            // should not have a default Content-Type header set.
+            needs_content_type = false;
+            break :brk MimeType.other;
+        };
     };
 
     return .{ content_type, needs_content_type, content_type_needs_free };

test/regression/issue/21193.test.ts

@@ -0,0 +1,84 @@
+import { expect, test } from "bun:test";
+
+// https://github.com/oven-sh/bun/issues/21193
+// Per the Fetch spec, BufferSource bodies (ArrayBuffer, TypedArray, DataView)
+// should not have a default Content-Type header.
+test("Response with Uint8Array body should not have content-type in Bun.serve", async () => {
+  using server = Bun.serve({
+    port: 0,
+    fetch() {
+      return new Response(new TextEncoder().encode("hello"));
+    },
+  });
+
+  const res = await fetch(server.url);
+  expect(res.headers.get("content-type")).toBeNull();
+  expect(await res.text()).toBe("hello");
+});
+
+test("Response with ArrayBuffer body should not have content-type in Bun.serve", async () => {
+  using server = Bun.serve({
+    port: 0,
+    fetch() {
+      return new Response(new ArrayBuffer(8));
+    },
+  });
+
+  const res = await fetch(server.url);
+  expect(res.headers.get("content-type")).toBeNull();
+  expect(res.headers.get("content-length")).toBe("8");
+});
+
+test("Response with DataView body should not have content-type in Bun.serve", async () => {
+  using server = Bun.serve({
+    port: 0,
+    fetch() {
+      return new Response(new DataView(new ArrayBuffer(4)));
+    },
+  });
+
+  const res = await fetch(server.url);
+  expect(res.headers.get("content-type")).toBeNull();
+  expect(res.headers.get("content-length")).toBe("4");
+});
+
+test("Response with string body should still have text/plain content-type", async () => {
+  using server = Bun.serve({
+    port: 0,
+    fetch() {
+      return new Response("hello");
+    },
+  });
+
+  const res = await fetch(server.url);
+  expect(res.headers.get("content-type")).toBe("text/plain;charset=utf-8");
+  expect(await res.text()).toBe("hello");
+});
+
+test("Response with Blob body should have blob content-type", async () => {
+  using server = Bun.serve({
+    port: 0,
+    fetch() {
+      return new Response(new Blob(["hello"], { type: "text/html" }));
+    },
+  });
+
+  const res = await fetch(server.url);
+  expect(res.headers.get("content-type")).toBe("text/html;charset=utf-8");
+  expect(await res.text()).toBe("hello");
+});
+
+test("Response with explicit content-type header and Uint8Array body should keep it", async () => {
+  using server = Bun.serve({
+    port: 0,
+    fetch() {
+      return new Response(new TextEncoder().encode("hello"), {
+        headers: { "content-type": "text/plain" },
+      });
+    },
+  });
+
+  const res = await fetch(server.url);
+  expect(res.headers.get("content-type")).toBe("text/plain");
+  expect(await res.text()).toBe("hello");
+});