fix case id, error if type is not guessed

[dadokkio]

Some fixes to virustotal download reponder.
If the type was not guessed properly it returned error, now keep the original hash as filename.
The case id was taken from case_id = self.get_param('data._parent') but that was None since the original input don't have that parameter. Now is taken from case_id = self.get_param("data.case._id")

Input eg:

Output eg:

@hariomenkel , since you wrote original code can you please check if everything is ok? thanks