searching for http upload server on github i found
successfully tested on python3
unknown support for uploading large files
will listen on all interfaces by default, allows to set --bind <host> <port> on CLI
meets the "Stores files on Amazon S3" criterium
While a bit bloated if the only thing you want is uploads, I eventually used Nextcloud for this; You can do this in nextcloud by creating a folder and setting up a share link for that folder with the permissions set to "Upload only" or "Edit".
Zooming out a bit and not technically answering the question, sending files can also be ...