NEXUS-7614: Fix NPE during attachment parsing

This merely fixes the symptom, but still unsure why
_attachments would contain any "non standard" fields
and not the field with content.

Code assumed file (hence content) is always present,
this change improves it to not expect that.

Issue
https://issues.sonatype.org/browse/NEXUS-7614

CI
http://bamboo.s/browse/NX-NPMF20