Edit: I found the solution! All I had to do was add the uid with my username, then I also had to add "forceuid" for it to actually go through. My fstab entry now looks like:

//192.168.1.21/Media-Library /mnt/Home-NAS/Media-Library cifs user=Jellyfin,password=password,uid=my_uid,forceuid,iocharset=utf8 0 0

Thank you @lemmyreader@lemmy.ml for posting the solution from Stack Exchange!

Hello! I have an Ubuntu server with a NAS mounted using cifs-utils, and I've created an entry in fstab for the share to be mounted at boot.

My fstab entry looks like this:

//192.168.1.21/Media-Library /mnt/Home-NAS/Media-Library cifs user=Jellyfin,password=password,iocharset=utf8 0 0

(The password is not actually "password" of course)

However, while I'm able to access the share perfectly fine, and even have a Jellyfin server reading from it, I cannot write files to the share without using sudo. I have some applications that manage metadata for music, and they're not able to change or add files in any way.

I am however able to access the share from my Fedora machine just fine with the same credentials, since I use KDE, I just added them to the default "Windows Share Credentials" setting. I don't have the issue where I have to use sudo to modify files, so I know it's just an issue with the share mounted to the server and not permission issues on the NAS itself.

What am I doing wrong?