diff --git a/src/index.js b/src/index.js
index f16e4635f..de45f0374 100644
--- a/src/index.js
+++ b/src/index.js
@@ -21,7 +21,9 @@ export default function wdm(compiler, options = {}) {
   if (mimeTypes) {
     const { types } = mime;
 
-    mime.types = { ...mimeTypes, ...types };
+    // mimeTypes from user provided options should take priority
+    // over existing, known types
+    mime.types = { ...types, ...mimeTypes };
   }
 
   const context = {
diff --git a/src/middleware.js b/src/middleware.js
index c9f09e7eb..c207efa9a 100644
--- a/src/middleware.js
+++ b/src/middleware.js
@@ -49,15 +49,9 @@ export default function wrapper(context) {
         return;
       }
 
-      try {
-        content = context.outputFileSystem.readFileSync(filename);
-      } catch (_ignoreError) {
-        await goNext();
-        return;
-      }
-
-      // Buffer
-      content = handleRangeHeaders(content, req, res);
+      // Content-Type and headers need to be set before checking if
+      // the file is in the outputFileSystem, as these things should be
+      // applied to all files that are being served
 
       if (!res.get('Content-Type')) {
         // content-type name(like application/javascript; charset=utf-8) or false
@@ -74,6 +68,16 @@ export default function wrapper(context) {
         }
       }
 
+      try {
+        content = context.outputFileSystem.readFileSync(filename);
+      } catch (_ignoreError) {
+        await goNext();
+        return;
+      }
+
+      // Buffer
+      content = handleRangeHeaders(content, req, res);
+
       // send Buffer
       res.send(content);
     }
diff --git a/test/middleware.test.js b/test/middleware.test.js
index b01c0a9b2..caa55261f 100644
--- a/test/middleware.test.js
+++ b/test/middleware.test.js
@@ -1990,7 +1990,7 @@ describe('middleware', () => {
 
       afterAll(close);
 
-      it('should return the "200" code for the "GET" request to "file.html"', (done) => {
+      it('should return the "200" code for the "GET" request to "file.phtml"', (done) => {
         request(app)
           .get('/file.phtml')
           .expect('Content-Type', 'application/octet-stream')
@@ -2031,13 +2031,91 @@ describe('middleware', () => {
 
       afterAll(close);
 
-      it('should return the "200" code for the "GET" request "file.html"', (done) => {
+      it('should return the "200" code for the "GET" request "file.phtml"', (done) => {
         request(app)
           .get('/file.phtml')
           .expect('Content-Type', 'text/html; charset=utf-8')
           .expect(200, 'welcome', done);
       });
     });
+
+    describe('should override value for "Content-Type" header for known MIME type', () => {
+      beforeAll((done) => {
+        const outputPath = path.resolve(__dirname, './outputs/basic');
+        const compiler = getCompiler({
+          ...webpackConfig,
+          output: {
+            filename: 'bundle.js',
+            path: outputPath,
+          },
+        });
+
+        instance = middleware(compiler, {
+          mimeTypes: {
+            jpg: 'application/octet-stream',
+          },
+        });
+
+        app = express();
+        app.use(instance);
+
+        listen = listenShorthand(done);
+
+        instance.context.outputFileSystem.mkdirSync(outputPath, {
+          recursive: true,
+        });
+        instance.context.outputFileSystem.writeFileSync(
+          path.resolve(outputPath, 'file.jpg'),
+          'welcome'
+        );
+      });
+
+      afterAll(close);
+
+      it('should return the "200" code for the "GET" request "file.jpg"', (done) => {
+        request(app)
+          .get('/file.jpg')
+          .expect('Content-Type', /application\/octet-stream/)
+          .expect(200, done);
+      });
+    });
+
+    describe('should set "Content-Type" header for route not from outputFileSystem', () => {
+      beforeAll((done) => {
+        const outputPath = path.resolve(__dirname, './outputs/basic');
+        const compiler = getCompiler({
+          ...webpackConfig,
+          output: {
+            filename: 'bundle.js',
+            path: outputPath,
+          },
+        });
+
+        instance = middleware(compiler, {
+          mimeTypes: {
+            jpg: 'application/octet-stream',
+          },
+        });
+
+        app = express();
+        app.use(instance);
+
+        app.get('/file.jpg', (req, res) => {
+          res.send('welcome');
+        });
+
+        listen = listenShorthand(done);
+      });
+
+      afterAll(close);
+
+      it('should return the "200" code for the "GET" request "file.jpg"', (done) => {
+        request(app)
+          .get('/file.jpg')
+          .expect('Content-Type', /application\/octet-stream/)
+          .expect(200, done);
+      });
+    });
   });
 
   describe('watchOptions option', () => {
@@ -2640,7 +2718,7 @@ describe('middleware', () => {
   });
 
   describe('headers option', () => {
-    beforeAll((done) => {
+    beforeEach((done) => {
       const compiler = getCompiler(webpackConfig);
 
       instance = middleware(compiler, {
@@ -2653,7 +2731,7 @@ describe('middleware', () => {
       listen = listenShorthand(done);
     });
 
-    afterAll(close);
+    afterEach(close);
 
     it('should return the "200" code for the "GET" request to the bundle file and return headers', (done) => {
       request(app)
@@ -2662,6 +2740,17 @@ describe('middleware', () => {
         .expect('X-nonsense-2', 'no')
         .expect(200, done);
     });
+
+    it('should return the "200" code for the "GET" request to path not in outputFileSystem and return headers', (done) => {
+      app.get('/file.jpg', (req, res) => {
+        res.send('welcome');
+      });
+      request(app)
+        .get('/file.jpg')
+        .expect('X-nonsense-1', 'yes')
+        .expect('X-nonsense-2', 'no')
+        .expect(200, done);
+    });
   });
 
   describe('publicPath option', () => {