When a picture is encrypted, save it to a temporary file first so that
the PHP function for rotation can access it as file.

Images on public sharing get downscaled to increase use experience - this will speed up loading time

- adding keep aspect to core/ajax/preview.php
- remove duplicate method Preview::show()
- no more hard coded mimetype of preview
- remove .png from the preview urls
- keep old route preview.png for backwards compatibility
- aspect preserving previews are now cached

This prevents excessive logging and disk access

Conflicts:
	lib/private/vcategories.php

This reverts commit 14a35267.