I am already using NFS for mounting backup here : Mount /home/yunohost.backup on a remote server - #6 by jarod5001
yunohost.multimedia/share should already have 775 as permissions. To fix this you need to mount using the correct uid and gid.
On your server check the id your_username
Use the correct id reported by the previous command in /etc/exports
/home/yunohost.multimedia your_laptop_ip(rw,sync,all_squash,anonuid=12345,anongid=12345)
Change 12345 with the correct id. Apply the changes et retry.