What are some alternatives?
When comparing tus.io and Filestack, you can also consider the following products
Uppy - The next open source file uploader for web browsers
Uploadcare - CDN, content delivery, content delivery network, file handling, image processing, cloud storage.
Uppload - File uploading widget with 20+ plugins, no backend required
Transloadit - Versatile file uploading and processing service for developers
CarrierWave - Solution for file uploads for Rails, Sinatra and other Ruby web frameworks.
Cloudinary - Cloudinary is a cloud-based service for hosting videos and images designed specifically with the needs of web and mobile developers in mind.