Default implementation of the filename generator strategy. It evaluates an
expression against the Message in order to generate the file name. Either
the 'expression' property can be set directly, or for a simple header name
to be used as the filename, there is also a setHeaderName(String)
method for convenience. If neither a header name nor custom expression is set,
the default header name is defined by the constant FileHeaders.FILENAME.
If no String-typed value is returned from the expression evaluation (or
associated with the header if no expression has been provided), it checks if
the Message payload is a File instance, and if so, it uses the same name.
Finally, it falls back to the Message ID and adds the suffix '.msg'.