When you attach a file to a bug in Bugzilla, Bugzilla automatically adds something like this to the start of the associated comment:
Created an attachment (id=11737)
When you're inside Bugzilla, it linkifies that so it's a hyperlink to the attachment data.
Jira doesn't do anything special with this text during import, so it becomes less useful in the imported data set. I would suggest that at a minimum, the ID number could be rewritten, because Jira gives the attachment a new ID. And a further extension (not sure how feasible this is though) would be to rewrite it into the wiki renderer syntax for a hyperlink to the attachment, so it functions exactly like the original comment in Bugzilla.