We have this today, it's just that this site doesn't support it. Only place I've seen this used (other than thumbnailing images that are drag and dropped on imgur and whatnot) is on Mega.
The MDN article mentions something which I have a question about;
>The API doesn't give you access to the local file system, nor is the sandbox really a section of the file system. Instead, it is a virtualized file system that looks like a full-fledged file system to the web app. It does not necessarily have a relationship to the local file system outside the browser.
>What this means is that a web app and a desktop app cannot share the same file at the same time. The API does not let your web app reach outside the browser to files that desktop apps can also work on. You can, however, export a file from a web app to a desktop app. For example, you can use the File API, create a blob, redirect an iframe to the blob, and invoke the download manager.
Is it possible to "see" from JavaScript whether or not the download manager has downloaded the file completely, so that the file can be removed from FileAPI storage?
Unfortunately, this is a non-standard API implemented only by Chrome. You have to use other APIs like IndexedDB and WebSQL (also deprecated and non-standard) to get a working solution in all browsers.
You can then use idb.filesystem.js to add api support for firefox etc. Search the file above for "is_chrome" for a few idb.filesystem.js-specific quirks.
Looking at that page, it looks like firefox will ship with support in version 50?
I'm using idb.filesystem.js in https://www.sharedrop.io, so that only very small part of the transferred file is stored in memory, but then without asking users for permission (i.e. using non-persistent storage) you "only" get ~4GB (not sure exactly, I tested it with files up to 1.5GB).
Have you looked into using IndexedDB? It's pretty widely supported [1] and far more performant than Files API last time I used it intensively (few years ago).
