How to work with the user's disk through WEB-technologies?

Question

Data synchronization

The task arose: to create an interface for the user, through which he can, both inside the corporate network and outside it on the entire range of personal devices (Win / OS X / Linux / Android / iOS), synchronize data between the server and the user's flash drive with all sorts of goodies unstable connection and little security.
The thought immediately flashed through my head that since it is possible to make Atom and VSCode, it is possible to somehow do it right in the browser (offline work is not needed).
I would be grateful if you can confirm my guesses and throw links where to read and where to look.

Answer 1

Why develop what is already ready?
Syncthing, Seafile, Nextcloud, Resilio Sync

Answer 2

HTML5 FileSystem API
HTML: JS:

var files;
document.getElementById("filepicker").addEventListener("change", function(event) {
  files = event.target.files;
}, false);

PS: Alternatively, you can use www.netdrive.net (cloud storage as a local disk)
I use it myself (I bought a life-time license) - very convenient!