Add ability to get tempfile to work with it

Method "to_file" of Carrierwave::SanitizedFile returns a file.
https://github.com/carrierwaveuploader/carrierwave/blob/master/lib/carrierwave/sanitized_file.rb#L234-L237
Sometimes it really needs to work with file.
So, with ftp or sftp we can use object of Tempfile, which looks like File, to work with it.
For this I wrote method 'to_file', which returns file.
If your project has several environments: in production used ftp, in develop - usual file storage, then you can use method 'to_file' to return file to work with it in any environment like with a file.