fix(ui): ensure file field is only serialized at top-level for upload-enabled collections (#12074)

This fixes an issue where fields with the name `file` was being
serialized as a top-level field in multipart form data even when the
collection was not upload-enabled. This caused the value of `file` (when
used as a regular field like a text, array, etc.) to be stripped from
the `_payload`.

- Updated `createFormData` to only delete `data.file` and serialize it
at the top level if `docConfig.upload` is defined.
- This prevents unintended loss of `file` field values for non-upload
collections.

The `file` field now remains safely nested in `_payload` unless it's
part of an upload-enabled collection.

test/admin/payload-types.ts

@@ -247,6 +247,7 @@ export interface Post {
   sidebarField?: string | null;
   wavelengths?: ('fm' | 'am') | null;
   selectField?: ('option1' | 'option2')[] | null;
+  file?: string | null;
   updatedAt: string;
   createdAt: string;
   _status?: ('draft' | 'published') | null;
@@ -665,6 +666,7 @@ export interface PostsSelect<T extends boolean = true> {
   sidebarField?: T;
   wavelengths?: T;
   selectField?: T;
+  file?: T;
   updatedAt?: T;
   createdAt?: T;
   _status?: T;