Unset Content-Type for FormData as request body before Request() constructor call

.changeset/nasty-squids-collect.md

@@ -0,0 +1,5 @@
+---
+"openapi-fetch": patch
+---
+
+Fix 'Content-Type' header being removed from requests with multipart/form-data body

packages/openapi-fetch/src/index.js

@@ -65,14 +65,14 @@ export default function createClient(clientOptions) {
     if (requestInit.body) {
       requestInit.body = bodySerializer(requestInit.body);
     }
+    // remove `Content-Type` if serialized body is FormData; browser will correctly set Content-Type & boundary expression
+    if (requestInit.body instanceof FormData) {
+      requestInit.headers.delete("Content-Type");
+    }
     let request = new Request(
       createFinalURL(url, { baseUrl, params, querySerializer }),
       requestInit,
     );
-    // remove `Content-Type` if serialized body is FormData; browser will correctly set Content-Type & boundary expression
-    if (requestInit.body instanceof FormData) {
-      request.headers.delete("Content-Type");
-    }
     // middleware (request)
     const mergedOptions = {
       baseUrl,

packages/openapi-fetch/test/index.test.ts

@@ -937,9 +937,9 @@ describe("client", () => {
       const req = fetchMocker.mock.calls[0][0];
       // note: this is FormData, but Node.js doesn’t handle new Request() properly with formData bodies. So this is only in tests.
       expect(req.body).toBeInstanceOf(Buffer);
-
-      // TODO: `vitest-fetch-mock` does not add the boundary to the Content-Type header like browsers do, so we expect the header to be null instead
-      expect(req.headers.get("Content-Type")).toBeNull();
+      expect((req.headers as Headers).get("Content-Type")).toBe(
+        "text/plain;charset=UTF-8",
+      );
     });
 
     // Node Requests eat credentials (no cookies), but this works in frontend

packages/openapi-fetch/test/v7-beta.test.ts

@@ -946,9 +946,9 @@ describe("client", () => {
       const req = fetchMocker.mock.calls[0][0];
       // note: this is FormData, but Node.js doesn’t handle new Request() properly with formData bodies. So this is only in tests.
       expect(req.body).toBeInstanceOf(Buffer);
-
-      // TODO: `vitest-fetch-mock` does not add the boundary to the Content-Type header like browsers do, so we expect the header to be null instead
-      expect((req.headers as Headers).get("Content-Type")).toBeNull();
+      expect((req.headers as Headers).get("Content-Type")).toBe(
+        "text/plain;charset=UTF-8",
+      );
     });
 
     // Node Requests eat credentials (no cookies), but this works in frontend