fix(fetch-adapter): set correct Content-Type for Node FormData (#6998)

* fix(fetch-adapter): set correct Content-Type for Node FormData

* Update lib/helpers/resolveConfig.js

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* test(fetch): replace chai expect with Node assert

* fix: define formHeaders for FormData to resolve no-undef error

* fix: filter headers to only update the target headers

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: Jay <jasonsaayman@gmail.com>

lib/helpers/resolveConfig.js

@@ -23,15 +23,19 @@ export default (config) => {
     );
   }
 
-  let contentType;
-
   if (utils.isFormData(data)) {
     if (platform.hasStandardBrowserEnv || platform.hasStandardBrowserWebWorkerEnv) {
-      headers.setContentType(undefined); // Let the browser set it
-    } else if ((contentType = headers.getContentType()) !== false) {
-      // fix semicolon duplication issue for ReactNative FormData implementation
-      const [type, ...tokens] = contentType ? contentType.split(';').map(token => token.trim()).filter(Boolean) : [];
-      headers.setContentType([type || 'multipart/form-data', ...tokens].join('; '));
+      headers.setContentType(undefined); // browser handles it
+    } else if (utils.isFunction(data.getHeaders)) {
+      // Node.js FormData (like form-data package)
+      const formHeaders = data.getHeaders();
+      // Only set safe headers to avoid overwriting security headers
+      const allowedHeaders = ['content-type', 'content-length'];
+      Object.entries(formHeaders).forEach(([key, val]) => {
+        if (allowedHeaders.includes(key.toLowerCase())) {
+          headers.set(key, val);
+        }
+      });
     }
   }  
 

test/unit/adapters/fetch.js

@@ -393,4 +393,20 @@ describe('supports fetch with nodejs', function () {
 
     assert.strictEqual(headers.get('foo'), 'bar');
   });
+
+  describe('fetch adapter - Content-Type handling', function () {
+    it('should set correct Content-Type for FormData automatically', async function () {
+      const FormData = (await import('form-data')).default; // Node FormData
+      const form = new FormData();
+      form.append('foo', 'bar');
+  
+      server = await startHTTPServer((req, res) => {
+        const contentType = req.headers['content-type'];
+        assert.match(contentType, /^multipart\/form-data; boundary=/i);
+        res.end('OK');
+      });
+  
+      await fetchAxios.post('/form', form);
+    });
+  });  
 });