Platform-specific file transmission
Override this method in subclasses to support platform-specific
file transmission. It is only called if the application's
return iterable ('self.result') is an instance of
'self.wsgi_file_wrapper'.
This method should return a true value if it was able to actually
transmit the wrapped file-like object using a platform-specific
approach. It should return a false value if normal iteration
should be used instead. An exception can be raised to indicate
that transmission was attempted, but failed.
NOTE: this method should call 'self.send_headers()' if
'self.headers_sent' is false and it is going to attempt direct
transmission of the file.