fixes download issue introduced by #10755

Conflicts:
	apps/files_sharing/public.php

Fixes https://github.com/owncloud/core/pull/11367#issuecomment-57306037 and https://github.com/owncloud/core/issues/9218

added tag to make facebook load a preview picture

Update public.php

add check ifMimeSupported and put the thumbsize in a variable

generate preview for all supported mimes

load image via javascript and use $(document).width() to determine the proper side of the image to be returned

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

- for supported previews are shown with a size of 500px; icons with a size of 128px

necessary for https://github.com/owncloud/apps/pull/1476