Attachment thumbnails cause slow loading times in issue view

Attaching images to the issue causes a significant performance drop. For ~50 images, loading times jump to ~18 sec.

I've noticed this on Jira 5.0.7, 5.1 and 5.1.5 (and probably happens to the versions in between).

This seems to be caused by the usage of ZipFile from apache.compress rather than java.util (which was used in 5.0.5) in AttachmentZipKit. (see attached image)

Using the java.util version of ZipFile dropped the loading times for an issue with ~150 images from 47s to 12s.