From f7777e79a52f475f51f894af5610a199396836f3 Mon Sep 17 00:00:00 2001
From: Apoorv Darshan <ad13dtu@gmail.com>
Date: Wed, 18 Feb 2026 21:36:12 +0530
Subject: [PATCH] fix: prevent res.set('Content-Type') from setting header to
 'false'

When mime.contentType() cannot resolve a shorthand value, it returns
false, which was silently set as the Content-Type header. Fall back to
the original value instead.

Fixes #7034
---
 lib/response.js |  2 +-
 test/res.set.js | 28 ++++++++++++++++++++++++++++
 2 files changed, 29 insertions(+), 1 deletion(-)

diff --git a/lib/response.js b/lib/response.js
index f965e539dd2..bc2bc6fb438 100644
--- a/lib/response.js
+++ b/lib/response.js
@@ -673,7 +673,7 @@ res.header = function header(field, val) {
       if (Array.isArray(value)) {
         throw new TypeError('Content-Type cannot be set to an Array');
       }
-      value = mime.contentType(value)
+      value = mime.contentType(value) || value
     }
 
     this.setHeader(field, value);
diff --git a/test/res.set.js b/test/res.set.js
index 04511c1c95f..de9cbff00dc 100644
--- a/test/res.set.js
+++ b/test/res.set.js
@@ -31,6 +31,34 @@ describe('res', function(){
       .expect('X-Number', '123')
       .expect(200, 'string', done);
     })
+
+    it('should preserve unknown Content-Type shorthand', function (done) {
+      var app = express();
+
+      app.use(function (req, res) {
+        res.set('Content-Type', 'custom-type');
+        res.end();
+      });
+
+      request(app)
+      .get('/')
+      .expect('Content-Type', 'custom-type')
+      .expect(200, done);
+    })
+
+    it('should set Content-Type with valid full MIME type', function (done) {
+      var app = express();
+
+      app.use(function (req, res) {
+        res.set('Content-Type', 'application/x-custom');
+        res.end();
+      });
+
+      request(app)
+      .get('/')
+      .expect('Content-Type', 'application/x-custom')
+      .expect(200, done);
+    })
   })
 
   describe('.set(field, values)', function(){