Userbase is a great open source solution: https://userbase.com/
This post has a bunch of other options that look viable: https://medium.com/visualdevspace/9-awesome-tools-you-can-use-to-build-membership-sites-e173e912c8a7
Headless CMSs like Ghost might offer something similar: https://ghost.org/vs/memberful/
The zoo of awesome backend frameworks that would perfectly suit your use case is huge.
Thus, what defines the choice is not so much the frameworks themselves but the availability of suitably trained/skilled people to you. See what skills you can get, and from there choose the tools.
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 ...
Alfresco ingests your documents (MS Office, LibrerOffice, PDF, text, etc), indexes them using Solr, and lets you search using a web interface.
It is free and open source (GPLv3 license), downloadable as Docker images.
It easily runs on a single machine. No coding needed.
Out-of-the-box it uses a file-based database which is not very powerful (but should be ...