Uploadcare - File uploading, media processing & content delivery for modern web apps
CarrierWave - Solution for file uploads for Rails, Sinatra and other Ruby web frameworks.
Filestack - Simple file uploader and robust APIs for uploading, transforming, and delivering any file into your app. Filestack is a collection of tools and powerful APIs that make it simple to upload, transform, and deliver content.
tus.io - File Uploads
Microsoft Azure Storage - Azure Storage is a durable, scalable cloud storage solution.
Uppy - The next open source file uploader for web browsers