fix: [stix export] Fixed email attachment export
fix: [stix export] Fixed email attachment export

- Fixed condition trying to reach the case where
  we have an attachment attribute in the object,
  which was never true because of a wrong key
  testing
- Fixed the email attachment related file object
  creation initiation

MISP/MISP,SteveClement/MISP,SteveClement/MISP,SteveClement/MISP,FIRSTdotorg/MISP,SteveClement/MISP,SteveClement/MISP,MISP/MISP,FIRSTdotorg/MISP,FIRSTdotorg/MISP,MISP/MISP,MISP/MISP,MISP/MISP,FIRSTdotorg/MISP