This denotes the range of subuids that are available to your user.
-u 100000:65536
This part specifies two things ([UID]:[GID]) even though it's the same syntax as the earlier part that specifies one range :)
I suspect what you will want to do is use the following:
# change ownership of the directory to the UID:GID that matches something in your subuid:subgid range, in this case 10000:10000
podman unshare chown -R 100000:10000 /home/privatenoob/media/storage1/Filmek/
Then we can specify that the user in the container can match the user (UID) we specified above:
As a note, if you copy/pasted that ExecStart line, you might have gotten the invalid argument error because you entered 100000 (outside of your subuid range, i.e. >65536) instead of 10000.
There's a nice guide that gives a great walkthrough. I'll dig through my bookmarks and add it here when I get some time.