From e08cbe6567ac8204f30df3af4a479e9e5eecc47a Mon Sep 17 00:00:00 2001
From: Harshit <harshit.singh22082007@gmail.com>
Date: Sun, 15 Mar 2026 17:13:32 +0530
Subject: [PATCH] fix: handle unknown Content-Type in res.set() falling back to
 original value

When mime.contentType() returns false for unrecognized types,
res.set('Content-Type', value) was setting the header to the
literal string 'false'. This change falls back to the original
value, matching how res.type() already handles this case.

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..44b7593ec38 100644
--- a/test/res.set.js
+++ b/test/res.set.js
@@ -87,6 +87,34 @@ describe('res', function(){
       .get('/')
       .expect(500, /TypeError: Content-Type cannot be set to an Array/, done)
     })
+
+    it('should fall back to original value for unknown Content-Type', function (done) {
+      var app = express();
+
+      app.use(function (req, res) {
+        res.set('Content-Type', 'some-custom-type');
+        res.end();
+      });
+
+      request(app)
+      .get('/')
+      .expect('Content-Type', 'some-custom-type')
+      .expect(200, done);
+    })
+
+    it('should resolve known Content-Type shorthands', function (done) {
+      var app = express();
+
+      app.use(function (req, res) {
+        res.set('Content-Type', 'html');
+        res.end();
+      });
+
+      request(app)
+      .get('/')
+      .expect('Content-Type', 'text/html; charset=utf-8')
+      .expect(200, done);
+    })
   })
 
   describe('.set(object)', function(){