Here’s another options:
You can create another project/endpoint in the parent domain. Probably if the parent domain already has a Wappler project, it would be just another SA which would handle file uploads.
On the sub domain, you can make use of API File Upload Action - NodeJS & PHP [Open Source] to upload file to the parent domain SA or API endpoint.
Although, having a central repository like S3 is a bit better if you don’t mind keeping the files on another server.