Hi.
there is a difference how the desktop apps and the mobile apps work.
To enable your OS to generate thumbnails, the content of a file has to be decrypted at some point.
With desktop apps your data is local available, and thus can be descrypted by cryptomator at any time.
The mobile apps connect directly to your cloud provider and only download the files you are requesting.
That means if you want to see thumbnails on your iOS, you have to download the file at least once so iOS can generate a thumbnail of it.