fix: [stix import] Fixed email attachments parsing
fix: [stix import] Fixed email attachments parsing

- Being less restrictive and supporting email
  attachments that are referencing objects not
  under the related object fields of the email,
  but referencing another object at the same level
  as the email, within the STIX incident
- Also parsing potential references even when the
  STIX file is coming from MISP export

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