Implement finalized interfaces #3

The actual final File API is quite different from the implementation of this library.

Namely, the File() constructor has a different signature:

new File(bits, name[, options])

https://developer.mozilla.org/en-US/docs/Web/API/File/File

I'd like to be able to have a consistent way to describe files across both client and server, so an up-to-date implementation of the File API as described on MDN (and implemented by modern browsers) would be necessary.

I'd be happy to implement this myself if you're open to a PR for it @coolaj86 @natlownes although it seems this repo is now dead, if I don't hear back from a maintainer that this work would be welcome I'll just create my own package from scratch, however I'd prefer to update this one to avoid polluting the NPM registry with similarly named packages having different implementations.