Bugfix: Setting image and readme for spaces

* An issue where setting a space-image or -readme would corrupt the file list has been fixed.
* Setting images from within the `.space` folder has been fixed.
* Setting images and readme files with specials characters in their names has been fixed.

https://github.com/owncloud/web/pull/6898
https://github.com/owncloud/web/issues/6875
