Fixes#35936.
When running `OC\Preview\BackgroundCleanupJob`, the main iteration loop
in `run()` expects a folder, however, `getOldPreviewLocations()`
currently does not filter by mimetype and therefore can yield a
non-folder entry which causes an Exception when constructing the Folder
impl.
Filtering for `httpd/unix-directory`, as `getNewPreviewLocations()`
already does, fixes this issue.
Signed-off-by: Dario Mehlich <d.mehlich@gmail.com>
654034638e