thats a timestamp, literally just the amount of seconds from Jan 1st, 1970. it's used by computers as a very simple way to tell time because it doesn't care about timezones, leap years, etc. sometimes it's written in milliseconds instead, this one is in seconds and resolves to Thu Jun 26 2025 10:47:20 (in GMT+0000)
4chan filenames are usually in milliseconds (sort of, the "milliseconds" part, so the last three digits, are actually random and not the real milliseconds, to try and prevent collisions, but collisions do sometimes still occur and will cause your post to get someone else's picture). as such, this is NOT a 4chan filename, but it does look similar. the 4chanX extension also has an option that automatically changes filenames in anything you post to be a random somewhat-recent timestamp for anonymity purposes.