Improvements: send filename to server. pass file data to callbacks.

1. Pass uploaded filename to /signS3put so that the server can return a namespaced filename such as "{12 character random}-{original filename}"

2. Remove unnecessary `decodeURIComponent` (#1) - just remove the unnecessary urlencode on the server side in your tutorial 

3. Pass filedata to the callbacks (so you can keep track of filesize, original filename, etc.)