Create TransientFile and file on FS from given input stream and 
        original file name.

def create_transient(self, input_stream, original_name, length=None):
        '''Create TransientFile and file on FS from given input stream and 
        original file name.'''
        ext = os.path.splitext(original_name)[1]
        transient = self.new_transient(ext)
        if not os.path.isdir(self.transient_root):
            os.makedirs(self.transient_root)

        self._copy_file(input_stream, transient.path, length=length)
        return transient