(WIP Answer)

Thanks to [muru's answer](https://unix.stackexchange.com/a/386880/143394) for the kickstart.

Using the information in `/proc/[pid]/fdinfo/[fd#]`:

 * Get mountpoint pathname and device using `mnt_id` as an index into `/proc/[pid]/mountinfo`
 * Convert `ino` to decimal
 * Lookup pathnames from inode:
 * [`btrfs`](https://serverfault.com/a/869356/322507) (requires pathname)
 * [`ext4`](https://unix.stackexchange.com/a/35310/143394) (requires device)