FORUM-885: [Forum] Incorrect attachment file name in Topic Form

Fix description:
* Correct the index to get attachment MIME type (the dot included)
FORUM-541: [Forum] UnsupportedOperationException when deleting or adding attachments

Problem analysis:
* During the update of a topic/post, the attachment cache data are inserted to topic/post object.
  These data are fixed-size Lists and therefore don't allow "add" or "remove" operation.

Fix description:
* Use addAll to get post information as resizable ArrayList.
FORUM-913 Paginator in forum doesn't work when not using Ajax option

Fix description:
* In non-ajax mode, use the URL in the href property instead of onclick property