fix: [stix import] Supporting multi attachment attributes for the email object
fix: [stix import] Supporting multi attachment attributes for the email object

- As specified in the object template

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