fix: update polyfills.js to include File (#8925)

* Update polyfills.js

* Update polyfills.js

* Changeset

* try this

---------

Co-authored-by: Rich Harris <[email protected]>

Author: ConProgramming

.changeset/famous-hornets-invite.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+chore: polyfill File from node:buffer

packages/kit/src/exports/node/polyfills.js

@@ -1,6 +1,10 @@
 import { ReadableStream, TransformStream, WritableStream } from 'node:stream/web';
+import buffer from 'node:buffer';
 import { webcrypto as crypto } from 'node:crypto';
-import { fetch, Response, Request, Headers, FormData } from 'undici';
+import { fetch, Response, Request, Headers, FormData, File as UndiciFile } from 'undici';
+
+// @ts-expect-error
+const File = buffer.File ?? UndiciFile;
 
 /** @type {Record<string, any>} */
 const globals = {
@@ -12,7 +16,8 @@ const globals = {
 	ReadableStream,
 	TransformStream,
 	WritableStream,
-	FormData
+	FormData,
+	File
 };
 
 // exported for dev/preview and node environments