Replace filepath with key in callback request

[stefanneculai]

I believe it is more useful to send the key instead of filepath to the callback_url. For example, I need the key to do additional processing on the server. Amazon also returns the key and probably for the same reason. I don't see any reason why not to return the data from Amazon (url and key) and return filepath instead.

Moreover, the current implementation has several issues in IE9:

• it was not returning the filepath, but only the path to the file. E.g. if the filepath would have been /uploads/foo/bar.jpg it was returning only /uploads/foo
• the filepath should start with a /. For filepaths such as /uploads/foo/bar.jpg it was returning only uploads/foo
• the filepath returned in IE9 was not encoded as it was in modern browsers.

[OleMchls]

👍 I was also in the need for the same. Not sure if this should remove existing fields, because BC. But I'd also to get key as a additional field!