File Handling provides a way for web applications to declare the ability to handle files with given MIME types and extensions. The web application will receive an event when the user intends to open a file with that web application.
Powerful web applications need to express their ability to read and edit files from the file system on the user’s device. Once a web application expresses this ability, it should be listed alongside native applications with similar abilities in host operating system surfaces, such as “Open with…” dialogs and context menus.
Docs: https://tinyurl.com/file-handling-design
Samples: https://principled-ring-yarrow.glitch.me
Explainers: https://github.com/WICG/file-handling/blob/master/explainer.md