chore: add changelog entry

changelog/unreleased/thumbnail-create-and-rename.md

@@ -0,0 +1,8 @@
+Bugfix: Fix possible race condition when a thumbnails is stored in the FS
+
+A race condition could cause the thumbnail service to return a thumbnail
+with 0 bytes or with partial content. In order to fix this, the service will
+create a temporary file with the contents and then rename that file to its
+final location.
+
+https://github.com/owncloud/ocis/pull/10693