Have there been any improvements in the area of importing attachments in versions 4.x?
Can the Bugzilla importer be used to upload already imported Work Items?

I have read most of the other links, but they are 1-3 years old at this stage and may not reflect the more recent versions of RTC, and improved capabilities, eg.
https://jazz.net/forum/questions/83329/how-to-import-attachments-into-rtc
https://jazz.net/library/article/69