public class AttachmentsCategoriser
Helper class for handling attachments on the view issue page. This class sorts attachments into two categories:
attachments that have thumbnails
attachments that do not have thumbnails
This is useful because of the different way in which these two types of attachments are displayed on the view issue
page. Note that an attachment may be "thumbnailable" but not have a thumbnail in practice, i.e. because there was an
error creating the thumbnail. This class cares not about theoretical thumbnailability, but only about actual
presence/absence of a thumbnail.