Preload file constrains for directives on the client

[gsuess]

It would be good to allow the client to validate the file against the size and file-type before making a round trip to the meteor server, without having to duplicate the parameters in the directive server side and client side.

In a nutshell something along the lines of:

var uploader = new Slingshot.Upload("myDirective");

if (file.size <= uploader.maxSize)
   uploader.send(file);
else
   this.error.set("File is too large");

[gsuess]

Fixed with #35