File hosting service with simple API

I’m building a tool that lets a small number of users try out some different design parameters, partially using their own files. They will be frequently replacing the files with new versions to see how they look in my app.

So I need a service where:

1. Users can upload their own files (ideally, using a Dropbox-like desktop synchronising tool).
2. My web app can easily access the files, and ideally get folder listings. A simple API would be be great (compared to the complexity of the Google Drive API, authentication etc).

The total storage requirements are tiny (eg, 5MB). Probably only .json and .png files will be stored.

This not for any kind of long term file sharing. Multiple users need to be able to share a single folder. Ideally, one user would create a folder, get a secret URL, and that URL would be all that was needed, without passwords etc.