Adds new file only if before_add was successful

I used before_add to do some javascript validation and the problem is that after an invalid fille (not uploaded, but added to the current_files) the next successful upload does not trigger 's3_uploads_complete' just 's3_upload_complete' because there is still one, not uploaded, file in the current_files array.

If this is the intended behavior just ignore this pull request.

And thanks for the great gem.