Add "Approximate Filesize" or "Filesize" if possible in the API.

[PizzaSpark]

I would like to see an "Approximate Filesize" or "Filesize" feature added to the Cobalt API. This feature would provide an estimate of the filesize for given files. If it's just an approximate, it would be fine.

[lostdusty]

When cobalt knows the final file size, the header Content-Lenght will contain the file size.