diff --git a/src/index.js b/src/index.js
index 2a6ed75..7053f2d 100644
--- a/src/index.js
+++ b/src/index.js
@@ -1,5 +1,3 @@
-import { mimes } from "mrmime";
-
 import middleware from "./middleware.js";
 import getFilenameFromUrl from "./utils/getFilenameFromUrl.js";
 import ready from "./utils/ready.js";
@@ -199,16 +197,6 @@ const noop = () => {};
  * @returns {API<RequestInternal, ResponseInternal>} rspack dev middleware
  */
 function rdm(compiler, options = {}) {
-  const { mimeTypes } = options;
-
-  if (mimeTypes) {
-    // mrmime.lookup closes over the exported dictionary, so overrides
-    // need to update the object in place.
-    for (const [extension, type] of Object.entries(mimeTypes)) {
-      mimes[extension.toLowerCase()] = type;
-    }
-  }
-
   /**
    * @type {WithOptional<Context<RequestInternal, ResponseInternal>, "watching" | "outputFileSystem">}
    */
diff --git a/src/middleware.js b/src/middleware.js
index df36f72..0823973 100644
--- a/src/middleware.js
+++ b/src/middleware.js
@@ -1,7 +1,7 @@
 import path from "node:path";
 import { finished } from "node:stream";
 
-import { lookup, mimes } from "mrmime";
+import { mimes } from "mrmime";
 import rangeParser from "range-parser";
 
 import {
@@ -47,14 +47,14 @@ const UTF8_CHARSET_MIME_TYPES = new Set([
   "application/manifest+json",
 ]);
 
-mimes.usdz = "model/vnd.usdz+zip";
-
 /**
- * @param {string} filename filename or extension
+ * @param {string} extension file extension including the leading dot
+ * @param {Map<string, string>} mimeTypes instance MIME mappings
  * @returns {string | undefined} content type header value
  */
-function getContentType(filename) {
-  const mimeType = lookup(filename);
+function getContentType(extension, mimeTypes) {
+  const ext = extension.trim().slice(1).toLowerCase();
+  const mimeType = mimeTypes.get(ext) ?? mimes[ext];
 
   if (!mimeType) {
     return undefined;
@@ -172,6 +172,14 @@ const MAX_MAX_AGE = 31536000000;
  * @returns {import("./index.js").Middleware<Request, Response>} wrapper
  */
 function wrapper(context) {
+  const mimeTypes = new Map([["usdz", "model/vnd.usdz+zip"]]);
+
+  for (const [extension, type] of Object.entries(
+    context.options.mimeTypes || {},
+  )) {
+    mimeTypes.set(extension.toLowerCase(), type);
+  }
+
   return async function middleware(req, res, next) {
     /**
      * @param {NodeJS.ErrnoException=} err an error
@@ -733,7 +741,7 @@ function wrapper(context) {
       ) {
         removeResponseHeader(res, "Content-Type");
         // content-type name (like application/javascript; charset=utf-8) or false
-        const contentType = getContentType(path.extname(filename));
+        const contentType = getContentType(path.extname(filename), mimeTypes);
 
         // Only set content-type header if media type is known
         // https://tools.ietf.org/html/rfc7231#section-3.1.1.5
diff --git a/test/middleware.test.js b/test/middleware.test.js
index 63db620..e033731 100644
--- a/test/middleware.test.js
+++ b/test/middleware.test.js
@@ -12,7 +12,7 @@ import finalhandler from "finalhandler";
 import { Hono } from "hono";
 import koa from "koa";
 import memfs, { Volume, createFsFromVolume } from "memfs";
-import { lookup, mimes } from "mrmime";
+import { lookup } from "mrmime";
 import router from "router";
 import request from "supertest";
 
@@ -47,8 +47,6 @@ const UTF8_CHARSET_MIME_TYPES = new Set([
   "application/manifest+json",
 ]);
 
-mimes.usdz = "model/vnd.usdz+zip";
-
 async function startServer(name, app) {
   return new Promise((resolve, reject) => {
     if (name === "router") {
@@ -3907,6 +3905,45 @@ describe.each([
     });
 
     describe("mimeTypes option", () => {
+      it("should isolate MIME overrides between instances", async () => {
+        const defaultType = lookup("js");
+        const expectedTypes = [
+          "text/plain; charset=utf-8",
+          "text/html; charset=utf-8",
+          getContentTypeHeader(),
+        ];
+        const instances = [];
+
+        try {
+          for (const mimeTypes of [
+            { JS: "text/plain" },
+            { js: "text/html" },
+            undefined,
+          ]) {
+            instances.push(
+              await frameworkFactory(
+                name,
+                framework,
+                getCompiler(webpackConfig),
+                {
+                  mimeTypes,
+                },
+              ),
+            );
+          }
+
+          for (const [index, [, request]] of instances.entries()) {
+            const response = await request.get("/bundle.js").expect(200);
+            expect(response.headers["content-type"]).toBe(expectedTypes[index]);
+          }
+          expect(lookup("js")).toBe(defaultType);
+        } finally {
+          await Promise.all(
+            instances.map(([server, , instance]) => close(server, instance)),
+          );
+        }
+      });
+
       describe('should set the correct value for "Content-Type" header to known MIME type', () => {
         beforeAll(async () => {
           const outputPath = path.resolve(__dirname, "./outputs/basic");