Still, it looks like the right thing to do is let the filesystem do the filesystem's job. The filesystem should be normalizing unicode and enforceing the case-insensitivity and whatnot, but just the filesystem. Wrappers around it like whatever Nextcloud is doing should be treating the filenames as a dumb pile of bytes.
I'm afraid I don't follow. Who maintains this table and who consumes it? What if they're different entities? How do you prevent it from going out of sync with the file system when the user renames a file? Are you inventing your own file system here? How do you deal with existing file systems?
I assumed that you have a system where file management/synchronization happens strictly through a web interface, and files are not changed or renamed outside this system's knowledge. Under these preconditions, having such a mapping table frees the users from having to abide whatever restrictions the underlying file system places on valid file names.
