The best alternatives to Filestack are Uppy, Uploader Window, and Shubox. If these 3 options don't work for you, we've listed a few more alternatives below.
Uppy is a sleek, modular JavaScript file uploader that integrates seamlessly with any application. It’s fast, easy to use and lets you worry about more important problems than building a file uploader. - Fetch files from local disk, urls, Dropbox, Instagram, or snap with a camera - Preview and edit metadata - Upload, optionally process & encode
Add file upload capabilities to your website or projects in just 1 line of JS code! Supports direct uploading from Facebook, Dropbox, GDrive, via Link, via Image Search, Mic, Camera, Youtube, etc. You can save files directly to your S3 bucket or your Dropbox spaces account. In built image editor so users can edit, resize, apply filters to images.