Due to a change to the database schema in Bugzilla versions 2.22.2 and later, attachments are not imported into JIRA. For details please see JRA-12389.
The instructions below will assist you in migrating attachments from Bugzilla 3.0.3 into JIRA 3.12. These instructions are not supported by Atlassian.
The attached zip file (BugzillaImportBean.zip) includes a patched up
BugzillaImportBean.java from JIRA 3.12.2. Please move the following files from "
<JIRA installation directory>/atlassian-jira/WEB-INF/classes/com/atlassian/jira/util/" into a backup directory:
Then extract the contents of the attached zip file into this directory.
Based on my simple test against Bugzilla 3.0.3, after the JIRA restart the attachment import functionality should be enabled.
You will need to delete the already imported bugzilla issues and redo the process again in order to get the attachments imported into JIRA.