Multi file upload

This is a pull request into sha256-custom branch.
This is unnecessary. What you could do is generate `_id` yourself (using `Random.id()`), set it to `file._id` and add it to the dict of files (you have ID available) and then you call insert and this is it. On insert you then start preprocessing.

You can generate ID on the client without any fears because it is local collection anyway. So there is no issues to think about when generating ID on the client in advance. So you do even need callback anymore on insert. You just store file object away and do an insert. And this is it. :-)