MariaDB Concurrent Connections with Update Statement
You cannot have "concurrent" updates of a single row.  The best you can do is to make them fast.

If you don't need the value, do it in a single statement:

UPDATE user SET files = files + 1 WHERE id = 1


Also, be sure that autocommit=ON is configured.

This will easily handle 100 increments per second on a spinning drive.  If you need even more speed, then set

innodb_flush_log_at_trx_commit = 2


This will cut back significantly on disk hits, at the potential loss of data for up to a second (in the case of a power failure).

max_connections = 25000 is grossly unreasonable; if you get more than a few dozen connections stumbling over each other, the system will appear to 'hang'.  At which point, more and more connections will be started, while the running connections run slower and slower.  This will because the OS is sharing the CPU, etc, among too many threads to actually get anything completed.

If you need thousands/second, I'll provide you with some other ideas.  (Note:  I say "per second", not "concurrently".)