fix: attachment sending (#11015)

* fix: attachment sending

* test: add tests

---------

Co-authored-by: Jiralite <33201955+Jiralite@users.noreply.github.com>

packages/rest/src/strategies/undiciRequest.ts

@@ -1,7 +1,7 @@
 import { STATUS_CODES } from 'node:http';
 import { URLSearchParams } from 'node:url';
 import { types } from 'node:util';
-import { type RequestInit, request, Headers } from 'undici';
+import { type RequestInit, request, Headers, FormData as UndiciFormData } from 'undici';
 import type { HeaderRecord } from 'undici/types/header.js';
 import type { ResponseLike } from '../shared.js';
 
@@ -52,8 +52,10 @@ export async function resolveBody(body: RequestInit['body']): Promise<Exclude<Re
 		return new Uint8Array(body.buffer);
 	} else if (body instanceof Blob) {
 		return new Uint8Array(await body.arrayBuffer());
-	} else if (body instanceof FormData) {
+	} else if (body instanceof UndiciFormData) {
 		return body;
+	} else if (body instanceof FormData) {
+		return globalToUndiciFormData(body);
 	} else if ((body as Iterable<Uint8Array>)[Symbol.iterator]) {
 		const chunks = [...(body as Iterable<Uint8Array>)];
 
@@ -70,3 +72,17 @@ export async function resolveBody(body: RequestInit['body']): Promise<Exclude<Re
 
 	throw new TypeError(`Unable to resolve body.`);
 }
+
+function globalToUndiciFormData(fd: globalThis.FormData): UndiciFormData {
+	const clone = new UndiciFormData();
+
+	for (const [name, value] of fd.entries()) {
+		if (typeof value === 'string') {
+			clone.append(name, value);
+		} else {
+			clone.append(name, value, value.name);
+		}
+	}
+
+	return clone;
+}