Fix handling of input of type FormData for the Fetch client

.changeset/loud-dancers-shake.md

@@ -0,0 +1,19 @@
+---
+"swagger-typescript-api": patch
+---
+
+Fix handling of FormData inputs in Fetch HTTP client
+
+Previously, when users passed a `FormData` object directly to the Fetch
+client's `multipart/form-data` formatter, it would incorrectly attempt to use
+`Object.keys()` on the FormData instance, which returns an empty array. This
+caused the FormData to be processed incorrectly.
+
+The fix adds a type check to return FormData instances unchanged, allowing
+users to have full control over FormData construction when needed whilst
+maintaining backwards compatibility for object inputs. This aligns the Fetch
+client behaviour with the existing Axios client implementation.
+
+This resolves issues where users needed to send multipart requests with
+multiple entries for the same key, which is only possible with direct FormData
+manipulation.

templates/base/http-clients/fetch-http-client.ejs

@@ -106,8 +106,12 @@ export class HttpClient<SecurityDataType = unknown> {
         [ContentType.Json]: (input:any) => input !== null && (typeof input === "object" || typeof input === "string") ? JSON.stringify(input) : input,
         [ContentType.JsonApi]: (input:any) => input !== null && (typeof input === "object" || typeof input === "string") ? JSON.stringify(input) : input,
         [ContentType.Text]: (input:any) => input !== null && typeof input !== "string" ? JSON.stringify(input) : input,
-        [ContentType.FormData]: (input: any) =>
-            Object.keys(input || {}).reduce((formData, key) => {
+        [ContentType.FormData]: (input: any) => {
+            if (input instanceof FormData) {
+                return input;
+            }
+
+            return Object.keys(input || {}).reduce((formData, key) => {
                 const property = input[key];
                 formData.append(
                     key,
@@ -118,7 +122,8 @@ export class HttpClient<SecurityDataType = unknown> {
                     `${property}`
                 );
                 return formData;
-            }, new FormData()),
+            }, new FormData());
+        },
         [ContentType.UrlEncoded]: (input: any) => this.toQueryString(input),
     }