What data is held in the operating system's file descriptors?

Question

I've been working with assembly and have been working with file IO. From what I've learned, the process goes as follows. CPU makes a system call to the kernel to open a file ie "hello.txt". The kernel then finds that location in the filesystem (persistent memory), makes it accessible for read and/or write, and returns a file descriptor that uniquely identifies that file. From my understanding the file descriptor is an index for a table that stores file data. My question is: what data is stored? presumably storing the entire file data would get grossly memory expense for large files. Does it store file metadata like mime-type, encoding, etc? Or does it actually store the whole contents?